since is an assert, can you provide the terminal output as well when it crashes?

From ~/.xsession-errors file, after a recent failure:

kontact(2757)/libakonadi Akonadi::EntityTreeModelPrivate::monitoredItemChanged: Got a stale notification for an item which was already removed. 997719 "" 
[6971:7005:1015/075627:ERROR:socket_stream.cc(208)] Closing stream with result -100
kontact(2757)/libakonadi Akonadi::EntityListCache<T, FetchJob, FetchScope_>::processResult: "Item query returned empty result set" 
QObject::connect: Cannot connect (null)::accessibilityChanged(bool,QString) to Baloo::StorageDevices::slotAccessibilityChanged(bool,QString)
QObject::connect: Cannot connect (null)::accessibilityChanged(bool,QString) to Baloo::StorageDevices::slotAccessibilityChanged(bool,QString)
QObject::connect: Cannot connect (null)::accessibilityChanged(bool,QString) to Baloo::StorageDevices::slotAccessibilityChanged(bool,QString)
QObject::connect: Cannot connect (null)::accessibilityChanged(bool,QString) to Baloo::StorageDevices::slotAccessibilityChanged(bool,QString)
QXcbConnection: XCB error: 3 (BadWindow), sequence: 2036, resource id: 0, major code: 21 (ListProperties), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24615, resource id: 123732145, major code: 20 (GetProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24631, resource id: 123732145, major code: 15 (QueryTree), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24652, resource id: 8388733, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24655, resource id: 8388733, major code: 12 (ConfigureWindow), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24656, resource id: 8388733, major code: 25 (SendEvent), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24706, resource id: 0, major code: 21 (ListProperties), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24748, resource id: 8388733, major code: 15 (QueryTree), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 24790, resource id: 123732143, major code: 15 (QueryTree), minor code: 0
Failed to register device: "device id 'xrandr-Dell Inc.-DELL P2815Q-WJJGR449CK9M' already exists"
EDID ICC Profile already exists "/home/garry/.local/share/icc/edid-a7d3c88e50c034634b69cd8788a09ff7.icc"
EDID ICC Profile already exists "/home/garry/.local/share/icc/edid-a7d3c88e50c034634b69cd8788a09ff7.icc"
QXcbConnection: XCB error: 3 (BadWindow), sequence: 25015, resource id: 8388748, major code: 15 (QueryTree), minor code: 0
QObject::connect: invalid null parameter
QObject::connect: invalid null parameter
QObject::connect: invalid null parameter
QObject::connect: invalid null parameter
Failed to register device: "device id 'xrandr-Dell Inc.-DELL P2815Q-WJJGR449CK9M' already exists"
Failed to register device: "device id 'xrandr-Dell Inc.-DELL P2815Q-WJJGR449CK9M' already exists"
EDID ICC Profile already exists "/home/garry/.local/share/icc/edid-a7d3c88e50c034634b69cd8788a09ff7.icc"
EDID ICC Profile already exists "/home/garry/.local/share/icc/edid-a7d3c88e50c034634b69cd8788a09ff7.icc"
EDID ICC Profile already exists "/home/garry/.local/share/icc/edid-a7d3c88e50c034634b69cd8788a09ff7.icc"
kscreen.kded: Config does not have at least one screen enabled, WILL NOT save this config, this is not what user wants.
QPainter::begin: Paint device returned engine == 0, type: 2
QPainter::save: Painter not active
QPainter::translate: Painter not active
QPainter::scale: Painter not active
QPainter::worldTransform: Painter not active
QPainter::setPen: Painter not active
QPainter::setBrush: Painter not active

[snip many similar messages]

QPainter::setWorldTransform: Painter not active
QPainter::setOpacity: Painter not active
QPainter::setWorldTransform: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::end: Painter not active, aborted
nativeResourceForScreen: null screen
nativeResourceForScreen: null screen
nativeResourceForScreen: null screen
nativeResourceForScreen: null screen
nativeResourceForScreen: null screen
nativeResourceForScreen: null screen
nativeResourceForScreen: null screen
Invalid pixmap specified.
No frame loaded
No frame loaded
No frame loaded
No frame loaded
No frame loaded
No frame loaded
QXcbConnection: XCB error: 3 (BadWindow), sequence: 27763, resource id: 123732232, major code: 15 (QueryTree), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 28205, resource id: 123731973, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 35599, resource id: 109051925, major code: 18 (ChangeProperty), minor code: 0

System log at time of crash:

oct 15 20:57:04 vfr kernel: usb 2-6: USB disconnect, device number 15
oct 15 20:57:05 vfr kernel: usb 2-6.2: USB disconnect, device number 16
oct 15 20:57:04 vfr acpid[873]: input device has been disconnected, fd 13
oct 15 20:57:05 vfr kernel: usb 2-6: new high-speed USB device number 17 using ehci-pci
oct 15 20:57:06 vfr kernel: [drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 163
oct 15 20:57:06 vfr kernel: [drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 163
oct 15 20:57:06 vfr kernel: usb 2-6: New USB device found, idVendor=0451, idProduct=8044
oct 15 20:57:06 vfr kernel: usb 2-6: New USB device strings: Mfr=0, Product=0, SerialNumber=1
oct 15 20:57:06 vfr kernel: usb 2-6: SerialNumber: 31020849D31E
oct 15 20:57:06 vfr kernel: hub 2-6:1.0: USB hub found
oct 15 20:57:06 vfr kernel: hub 2-6:1.0: 4 ports detected
oct 15 20:57:06 vfr kernel: usb 2-6.2: new full-speed USB device number 18 using ehci-pci
oct 15 20:57:06 vfr kernel: usb 2-6.2: New USB device found, idVendor=413c, idProduct=a503
oct 15 20:57:06 vfr kernel: usb 2-6.2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
oct 15 20:57:06 vfr kernel: usb 2-6.2: Product: Dell AC511 USB SoundBar
oct 15 20:57:06 vfr kernel: usb 2-6.2: Manufacturer: Dell
oct 15 20:57:06 vfr kernel: input: Dell Dell AC511 USB SoundBar as /devices/pci0000:00/0000:00:1d.7/usb2/2-6/2-6.2/2-6.2:1.3/0003:413C:A503.000E/input/input23
oct 15 20:57:06 vfr kernel: hid-generic 0003:413C:A503.000E: input,hidraw5: USB HID v1.00 Device [Dell Dell AC511 USB SoundBar] on usb-0000:00:1d.7-6.2/input3
oct 15 20:57:07 vfr kernel: [drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 150
oct 15 20:57:07 vfr kernel: Raw EDID:
oct 15 20:57:07 vfr kernel:          00 ff ff ff ff ff ff 00 10 ac 5c f0 4d 39 7f ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
oct 15 20:57:07 vfr kernel: [drm:radeon_dvi_detect [radeon]] *ERROR* HDMI-A-1: probed a monitor but no|invalid EDID
oct 15 20:57:07 vfr mtp-probe[18663]: checking bus 2, device 18: "/sys/devices/pci0000:00/0000:00:1d.7/usb2/2-6/2-6.2"
oct 15 20:57:07 vfr mtp-probe[18663]: bus: 2, device: 18 was not an MTP device
oct 15 20:57:34 vfr cupsd[2747]: REQUEST localhost - - "POST / HTTP/1.1" 200 415 Create-Printer-Subscriptions successful-ok
oct 15 20:57:35 vfr dbus[812]: [system] Activating via systemd: service name='org.bluez' unit='dbus-org.bluez.service'
oct 15 20:57:35 vfr dbus[812]: [system] Activation via systemd failed for unit 'dbus-org.bluez.service': Unit dbus-org.bluez.service failed to load: No such file or directory.
oct 15 20:57:35 vfr dbus[812]: [system] Activating via systemd: service name='org.bluez' unit='dbus-org.bluez.service'
oct 15 20:57:35 vfr dbus[812]: [system] Activation via systemd failed for unit 'dbus-org.bluez.service': Unit dbus-org.bluez.service failed to load: No such file or directory.

OK, maybe this will help.

The crash happens almost all the time when the monitor is power-cycled.

I started plasma in a terminal that I also ran a journalctl -f command in, too.
So plasma's stdout is interspersed with the system log.

$ rpm -qa|grep plasma
plasma-milou-5.4.3-1.fc23.x86_64
plasma-workspace-5.4.3-2.fc23.x86_64
plasma-nm-openvpn-5.4.3-1.fc23.x86_64
plasma-breeze-common-5.4.3-2.fc23.noarch
plasma-workspace-libs-5.4.3-2.fc23.x86_64
plasma-workspace-common-5.4.3-2.fc23.x86_64
plasma-workspace-geolocation-libs-5.4.3-2.fc23.x86_64
kdeplasma-addons-5.4.3-1.fc23.x86_64
plasma-workspace-geolocation-5.4.3-2.fc23.x86_64
plasma-breeze-5.4.3-2.fc23.x86_64
plasma-desktop-5.4.3-1.fc23.x86_64
kf5-plasma-5.16.0-1.fc23.x86_64
plasma-nm-5.4.3-1.fc23.x86_64
plasma-pa-5.4.3-1.fc23.x86_64
plasma-nm-pptp-5.4.3-1.fc23.x86_64
plasma-workspace-drkonqi-5.4.3-2.fc23.x86_64
kde-settings-plasma-23-8.fc23.noarch
plasma-systemsettings-5.4.3-1.fc23.x86_64
plasma-nm-vpnc-5.4.3-1.fc23.x86_64
$

Here's a recent crash:

Nov 19 22:23:04 vfr kernel: usb 2-6: USB disconnect, device number 11
Nov 19 22:23:04 vfr kernel: usb 2-6.2: USB disconnect, device number 12
Nov 19 22:23:04 vfr acpid[806]: input device has been disconnected, fd 13
Nov 19 22:23:05 vfr kernel: usb 2-6: new high-speed USB device number 13 using ehci-pci
Nov 19 22:23:05 vfr kernel: usb 2-6: New USB device found, idVendor=0451, idProduct=8044
Nov 19 22:23:05 vfr kernel: usb 2-6: New USB device strings: Mfr=0, Product=0, SerialNumber=1
Nov 19 22:23:05 vfr kernel: usb 2-6: SerialNumber: 31020849D31E
Nov 19 22:23:05 vfr kernel: hub 2-6:1.0: USB hub found
Nov 19 22:23:05 vfr kernel: hub 2-6:1.0: 4 ports detected
Nov 19 22:23:05 vfr kernel: usb 2-6.2: new full-speed USB device number 14 using ehci-pci
Nov 19 22:23:05 vfr kernel: usb 2-6.2: New USB device found, idVendor=413c, idProduct=a503
Nov 19 22:23:05 vfr kernel: usb 2-6.2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
Nov 19 22:23:05 vfr kernel: usb 2-6.2: Product: Dell AC511 USB SoundBar
Nov 19 22:23:05 vfr kernel: usb 2-6.2: Manufacturer: Dell
Nov 19 22:23:05 vfr kernel: input: Dell Dell AC511 USB SoundBar as /devices/pci0000:00/0000:00:1d.7/usb2/2-6/2-6.2/2-6.2:1.3/0003:413C:A503.000C/input/input21
Nov 19 22:23:05 vfr kernel: hid-generic 0003:413C:A503.000C: input,hidraw4: USB HID v1.00 Device [Dell Dell AC511 USB SoundBar] on usb-0000:00:1d.7-6.2/input3
Nov 19 22:23:06 vfr mtp-probe[30946]: checking bus 2, device 14: "/sys/devices/pci0000:00/0000:00:1d.7/usb2/2-6/2-6.2"
Nov 19 22:23:06 vfr mtp-probe[30946]: bus: 2, device: 14 was not an MTP device
Nov 19 22:23:06 vfr kernel: [drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 28
Nov 19 22:23:06 vfr kernel: Raw EDID:
Nov 19 22:23:06 vfr kernel:          00 ff ff ff ff ff ff 00 10 ac 5c f0 4d 3f ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel:          ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
Nov 19 22:23:06 vfr kernel: [drm:radeon_dvi_detect [radeon]] *ERROR* HDMI-A-1: probed a monitor but no|invalid EDID
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 44342, resource id: 0, major code: 21 (ListProperties), minor code: 0
Application::crashHandler() called with signal 11; recent crashes: 1
/usr/bin/plasmashell  --crashes 1 &
KCrash: Application 'plasmashell' crashing...
KCrash: Attempting to start /usr/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
[1]  + suspended (signal)  plasmashell
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
No metadata file in the package, expected it at: "/usr/share/backgrounds/f23/default/tv-wide//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/backgrounds/f23/default/tv-wide//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/backgrounds/f23/default/tv-wide//metadata.desktop"
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemloadviewer/contents/ui/BarMonitor.qml:49:20: Unable to assign [undefined] to QColor
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemloadviewer/contents/ui/SystemLoadViewer.qml:68: TypeError: Cannot read property of null
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemloadviewer/contents/ui/SystemLoadViewer.qml:153: TypeError: Cannot read property 'value' of undefined
Known plasmoid ids: QHash(("org.kde.plasma.volume", 23)("org.kde.plasma.clipboard", 10)("org.kde.plasma.notifications", 12)("org.kde.plasma.devicenotifier", 11)("org.kde.plasma.printmanager", 13)("org.kde.plasma.battery", 14)("org.kde.plasma.networkmanagement", 15))
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
file:///usr/lib64/qt5/qml/org/kde/plasma/extras/ScrollArea.qml:48: Error: Cannot assign to non-existent property "interactive"
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollBar.qml:91: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollBar.qml:91: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:114: TypeError: Cannot read property 'corner' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:63: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:64: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:65: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:66: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/ScrollView.qml:286: TypeError: Cannot read property '__wheelScrollLines' of null
file:///usr/share/plasma/plasmoids/org.kde.plasma.clipboard/contents/ui/clipboard.qml:32: TypeError: Cannot read property 'empty' of undefined
file:///usr/share/plasma/plasmoids/org.kde.plasma.clipboard/contents/ui/clipboard.qml:34: TypeError: Cannot read property 'empty' of undefined
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollBar.qml:91: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollBar.qml:91: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:114: TypeError: Cannot read property 'corner' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:63: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:64: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:65: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Private/ScrollViewHelper.qml:66: TypeError: Cannot read property 'padding' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/ScrollView.qml:286: TypeError: Cannot read property '__wheelScrollLines' of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:106:17: Unable to assign [undefined] to QObject*
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:85: TypeError: Cannot read property 'effectivePressed' of undefined
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:106:17: Unable to assign [undefined] to QObject*
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:85: TypeError: Cannot read property 'effectivePressed' of undefined
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:106:17: Unable to assign [undefined] to QObject*
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:85: TypeError: Cannot read property 'effectivePressed' of undefined
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:106:17: Unable to assign [undefined] to QObject*
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null
file:///usr/lib64/qt5/qml/QtQuick/Controls/Button.qml:85: TypeError: Cannot read property 'effectivePressed' of undefined
Nov 19 22:23:31 vfr cupsd[1927]: REQUEST localhost - - "POST / HTTP/1.1" 200 415 Create-Printer-Subscriptions successful-ok
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
file:///usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/DigitalClock.qml:444:5: QML Text: Cannot anchor to a null item.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.printmanager/contents/ui/printmanager.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
Both point size and pixel size set. Using pixel size.
file:///usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/DigitalClock.qml:444:5: QML Text: Binding loop detected for property "height"
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemloadviewer/contents/ui/SystemLoadViewer.qml:179: TypeError: Cannot read property 'value' of undefined
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemloadviewer/contents/ui/SystemLoadViewer.qml:179: TypeError: Cannot read property 'value' of undefined
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemloadviewer/contents/ui/SystemLoadViewer.qml:179: TypeError: Cannot read property 'value' of undefined
networkmanager-qt: void NetworkManager::NetworkManagerPrivate::propertiesChanged(const QVariantMap&) Unhandled property "Devices"
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.networkmanagement/contents/ui/main.qml: QML Plasmoid: Cannot anchor to an item that isn't a parent or sibling.
file:///usr/share/plasma/plasmoids/org.kde.plasma.systemtray/contents/ui/StatusNotifierItem.qml:131:13: QML Image: Failed to get image from provider: image://icon/
[1]  + suspended (signal)  plasmashell

This happens to me too - all that is needed is to disconnect the monitor and plasma crashes every time. If it would be helpful I could try compiling kde with debugging symbols and send a report.

This happens for me as well on every monitor Off/On Cycle with version 5.4.3, it looks like a regression. It worked with earlier versions, not sure when exactly this started to happen though.

Useful console output:
...
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
requesting unexisting screen 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 2286, resource id: 0, major code: 21 (ListProperties), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 2336, resource id: 0, major code: 21 (ListProperties), minor code: 0

_(crash)_

gdb backtrace:

Thread 9 (Thread 0x7f0d5ab8d700 (LWP 2284)):
#0  0x00007f0d6ab4f20d in poll () from /lib64/libc.so.6
#1  0x00007f0d6fa0c432 in _xcb_conn_wait () from /usr/lib64/libxcb.so.1
#2  0x00007f0d6fa0e007 in xcb_wait_for_event () from /usr/lib64/libxcb.so.1
#3  0x00007f0d5ccdce29 in ?? () from /usr/lib64/libQt5XcbQpa.so.5
#4  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#5  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#6  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 8 (Thread 0x7f0d53dfa700 (LWP 2285)):
#0  0x00007f0d6ab4f20d in poll () from /lib64/libc.so.6
#1  0x00007f0d67246264 in ?? () from /usr/lib64/libglib-2.0.so.0
#2  0x00007f0d6724636c in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#3  0x00007f0d6b46550f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#4  0x00007f0d6b40f63a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#5  0x00007f0d6b23bb1c in QThread::exec() () from /usr/lib64/libQt5Core.so.5
#6  0x00007f0d6e4e79a5 in ?? () from /usr/lib64/libQt5Qml.so.5
#7  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#8  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#9  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 7 (Thread 0x7f0d46da6700 (LWP 2286)):
#0  0x00007f0d6ab4f20d in poll () from /lib64/libc.so.6
#1  0x00007f0d67246264 in ?? () from /usr/lib64/libglib-2.0.so.0
#2  0x00007f0d6724636c in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#3  0x00007f0d6b46550f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#4  0x00007f0d6b40f63a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#5  0x00007f0d6b23bb1c in QThread::exec() () from /usr/lib64/libQt5Core.so.5
#6  0x00007f0d6e4e79a5 in ?? () from /usr/lib64/libQt5Qml.so.5
#7  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#8  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#9  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 6 (Thread 0x7f0d4530c700 (LWP 2289)):
#0  0x00007f0d6ab4f20d in poll () from /lib64/libc.so.6
#1  0x00007f0d67246264 in ?? () from /usr/lib64/libglib-2.0.so.0
#2  0x00007f0d6724636c in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#3  0x00007f0d6b46550f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#4  0x00007f0d6b40f63a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#5  0x00007f0d6b23bb1c in QThread::exec() () from /usr/lib64/libQt5Core.so.5
#6  0x00007f0d6e4e79a5 in ?? () from /usr/lib64/libQt5Qml.so.5
#7  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#8  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#9  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 5 (Thread 0x7f0d3f99e700 (LWP 2290)):
#0  0x00007f0d6a35c07f in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007f0d70c7ba84 in ?? () from /usr/lib64/libQt5Script.so.5
#2  0x00007f0d70c7bac9 in ?? () from /usr/lib64/libQt5Script.so.5
#3  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#4  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 4 (Thread 0x7f0cb7fff700 (LWP 2291)):
#0  0x00007f0d6ab4f20d in poll () from /lib64/libc.so.6
#1  0x00007f0d67246264 in ?? () from /usr/lib64/libglib-2.0.so.0
#2  0x00007f0d6724636c in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#3  0x00007f0d6b46552b in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#4  0x00007f0d6b40f63a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#5  0x00007f0d6b23bb1c in QThread::exec() () from /usr/lib64/libQt5Core.so.5
#6  0x00007f0d6f069e42 in ?? () from /usr/lib64/libQt5Quick.so.5
#7  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#8  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#9  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

---Type <return> to continue, or q <return> to quit---
Thread 3 (Thread 0x7f0caba19700 (LWP 2293)):
#0  0x00007f0d6ab4f20d in poll () from /lib64/libc.so.6
#1  0x00007f0d67246264 in ?? () from /usr/lib64/libglib-2.0.so.0
#2  0x00007f0d6724636c in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#3  0x00007f0d6b46550f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#4  0x00007f0d6b40f63a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#5  0x00007f0d6b23bb1c in QThread::exec() () from /usr/lib64/libQt5Core.so.5
#6  0x00007f0cadf2ce47 in KCupsConnection::run() () from /usr/lib64/libkcupslib.so
#7  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#8  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#9  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 2 (Thread 0x7f0ca89de700 (LWP 2294)):
#0  0x00007f0d6a35c428 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007f0d6b241a06 in QWaitCondition::wait(QMutex*, unsigned long) () from /usr/lib64/libQt5Core.so.5
#2  0x00007f0d6b23d6ab in ?? () from /usr/lib64/libQt5Core.so.5
#3  0x00007f0d6b24094f in ?? () from /usr/lib64/libQt5Core.so.5
#4  0x00007f0d6a3564a4 in start_thread () from /lib64/libpthread.so.0
#5  0x00007f0d6ab57b9d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7f0d715a38c0 (LWP 2283)):
#0  0x00007f0d6b979320 in QScreen::handle() const () from /usr/lib64/libQt5Gui.so.5
#1  0x00007f0d5cce46e9 in QXcbIntegration::createPlatformWindow(QWindow*) const ()
   from /usr/lib64/libQt5XcbQpa.so.5
#2  0x00007f0d6b95c0d7 in QWindowPrivate::create(bool) () from /usr/lib64/libQt5Gui.so.5
#3  0x00007f0d6b95c368 in QWindow::winId() const () from /usr/lib64/libQt5Gui.so.5
#4  0x00007f0d70fa8d65 in ?? () from /usr/lib64/libKF5PlasmaQuick.so.5
#5  0x00007f0d70fab6d9 in ?? () from /usr/lib64/libKF5PlasmaQuick.so.5
#6  0x00007f0d6b43ef8a in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib64/libQt5Core.so.5
#7  0x00007f0d6b43ee17 in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib64/libQt5Core.so.5
#8  0x00007f0d6b43ef8a in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib64/libQt5Core.so.5
#9  0x00007f0d6b44b0e8 in QTimer::timerEvent(QTimerEvent*) () from /usr/lib64/libQt5Core.so.5
#10 0x00007f0d6b43fd2b in QObject::event(QEvent*) () from /usr/lib64/libQt5Core.so.5
#11 0x00007f0d6c7878cc in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
   from /usr/lib64/libQt5Widgets.so.5
#12 0x00007f0d6c78c9d6 in QApplication::notify(QObject*, QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#13 0x00007f0d6b411cf3 in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib64/libQt5Core.so.5
#14 0x00007f0d6b4643bd in QTimerInfoList::activateTimers() () from /usr/lib64/libQt5Core.so.5
#15 0x00007f0d6b4648d1 in ?? () from /usr/lib64/libQt5Core.so.5
#16 0x00007f0d67246097 in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#17 0x00007f0d672462c8 in ?? () from /usr/lib64/libglib-2.0.so.0
#18 0x00007f0d6724636c in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#19 0x00007f0d6b46550f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#20 0x00007f0d6b40f63a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib64/libQt5Core.so.5
#21 0x00007f0d6b4172fd in QCoreApplication::exec() () from /usr/lib64/libQt5Core.so.5
#22 0x0000000000434150 in main ()

can you reproduce and get the console output as well when the assert gets hit?

(In reply to Marco Martin from comment #7)
> can you reproduce and get the console output as well when the assert gets
> hit?

It is reproducible and the output is (already stated above):

Useful console output: 
... 
requesting unexisting screen 0 
requesting unexisting screen 0 
requesting unexisting screen 0 
requesting unexisting screen 0 
requesting unexisting screen 0 
requesting unexisting screen 0 
requesting unexisting screen 0 
requesting unexisting screen 0 
QXcbConnection: XCB error: 3 (BadWindow), sequence: 2286, resource id: 0, major code: 21 (ListProperties), minor code: 0 
QXcbConnection: XCB error: 3 (BadWindow), sequence: 2336, resource id: 0, major code: 21 (ListProperties), minor code: 0


Maybe it is important to say: This is a notebook connected to an external monitor, where the internal monitor is disabled, thus when disabling the external monitor, the last output goes away. _Maybe_ the unexisting screen 0 is the internal notebook lcd...

Try : In >Startup and Shutdown>Background Serivces>  untick "KScreen2" and reboot.

(In reply to e8hffff from comment #9)
> Try : In >Startup and Shutdown>Background Serivces>  untick "KScreen2" and
> reboot.

I'm not sure what you have in mind with this, this lets my system just "forget" my monitor setup. Applying the above mentioned monitor setup after your suggestion (disable and reboot) results in the same crash as noted in Comment #6.

*** This bug has been marked as a duplicate of bug 341497 ***