If something is crashing, I need a backtrace:

see https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports

(gdb) r
Starting program: /usr/bin/plasmashell 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffe1ca4700 (LWP 32369)]
[New Thread 0x7fffdb713700 (LWP 32370)]
[New Thread 0x7fffd9cce700 (LWP 32372)]
Aborting shell load: The activity manager daemon (kactivitymanagerd) is not running.
If this Plasma has been installed into a custom prefix, verify that its D-Bus services dir is known to the system for the daemon to be activatable.
org.kde.plasmaquick: Applet preload policy set to 1
[New Thread 0x7fffd3bb5700 (LWP 32377)]
[New Thread 0x7fffd181e700 (LWP 32378)]
Warning: all files used by qml by the plasmoid should be in ui/. The file in the path "file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/code/LayoutManager.js" was expected at QUrl("file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/LayoutManager.js")
Warning: all files used by qml by the plasmoid should be in ui/. The file in the path "file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/code/FolderTools.js" was expected at QUrl("file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/FolderTools.js")
[New Thread 0x7fff3d7e2700 (LWP 32386)]
trying to show an empty dialog
org.kde.plasmaquick: New Applet  "Application Launcher" with a weight of 100
Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)
Trying to use rootObject before initialization is completed, whilst using setInitializationDelayed. Forcing completion
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.
trying to show an empty dialog
org.kde.plasmaquick: New Applet  "Digital Clock" with a weight of 50
Warning: all files used by qml by the plasmoid should be in ui/. The file in the path "file:///usr/share/plasma/plasmoids/org.kde.panel/contents/code/LayoutManager.js" was expected at QUrl("file:///usr/share/plasma/plasmoids/org.kde.panel/contents/ui/LayoutManager.js")
trying to show an empty dialog
Failed to find service for Unity Launcher "application://Mailspring.desktop"
[New Thread 0x7fff2de39700 (LWP 32423)]
org.kde.plasmaquick: Delayed preload of  "Application Launcher" after 0 seconds
KAStatsFavoritesModel::setFavorites is ignored
KActivities: Database connection:  "kactivities_db_resources_140737353435136_readonly" 
    query_only:          QVariant(qlonglong, 1) 
    journal_mode:        QVariant(QString, "wal") 
    wal_autocheckpoint:  QVariant(qlonglong, 100) 
    synchronous:         QVariant(qlonglong, 0)
Entry is not valid "org.kde.kontact.desktop" 0x555557890270
Entry is not valid "org.kde.kontact.desktop" 0x555557824400
Closing SQL connection:  "kactivities_db_resources_140737353435136_readonly"
trying to show an empty dialog
org.kde.plasmaquick: Applet "Application Launcher" loaded after 343 msec
org.kde.plasmaquick: New Applet  "Audio Volume" with a weight of 50
org.kde.plasmaquick: New Applet  "Clipboard" with a weight of 50
org.kde.plasmaquick: New Applet  "Device Notifier" with a weight of 50
org.kde.plasmaquick: New Applet  "KDE Connect" with a weight of 50
Warning: all files used by qml by the plasmoid should be in ui/. The file in the path "file:///usr/share/plasma/plasmoids/org.kde.plasma.notifications/contents/code/uiproperties.js" was expected at QUrl("file:///usr/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/uiproperties.js")
org.kde.plasmaquick: New Applet  "Notifications" with a weight of 50
[New Thread 0x7fff2116b700 (LWP 32527)]
libkcups: Create-Printer-Subscriptions last error: 0 successful-ok
org.kde.plasmaquick: New Applet  "Printers" with a weight of 50
libkcups: Get-Jobs last error: 0 successful-ok
libkcups: Get-Jobs last error: 0 successful-ok
[New Thread 0x7fff1a3b2700 (LWP 32530)]
[New Thread 0x7fff19bb1700 (LWP 32531)]
[New Thread 0x7fff193b0700 (LWP 32532)]
[New Thread 0x7fff18baf700 (LWP 32533)]

(process:32335): OSTree-WARNING **: 20:39:49.331: Invalid proxy URI ''

(process:32335): OSTree-WARNING **: 20:39:49.331: Invalid proxy URI ''
[New Thread 0x7fff0bfff700 (LWP 32534)]
org.kde.plasmaquick: New Applet  "Updates" with a weight of 50
[New Thread 0x7fff0b7fe700 (LWP 32535)]
org.kde.plasmaquick: New Applet  "Vaults" with a weight of 50
Plasma Shell startup completed
[New Thread 0x7fff0addc700 (LWP 32536)]
[New Thread 0x7fff0a5db700 (LWP 32537)]
libkcups: 6

Thread 1 "plasmashell" received signal SIGSEGV, Segmentation fault.
0x00007fff23ae5556 in JobModel::updateJob(int, KCupsJob const&) ()
   from /usr/lib/x86_64-linux-gnu/libkcupslib.so
(gdb) bt
#0  0x00007fff23ae5556 in JobModel::updateJob(int, KCupsJob const&) ()
    at /usr/lib/x86_64-linux-gnu/libkcupslib.so
#1  0x00007fff23ae9e9f in JobModel::getJobFinished() () at /usr/lib/x86_64-linux-gnu/libkcupslib.so
#2  0x00007fff23aee3b5 in  () at /usr/lib/x86_64-linux-gnu/libkcupslib.so
#3  0x00007ffff1e8f0c2 in QObject::event(QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#4  0x00007ffff2e6783c in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#5  0x00007ffff2e6f104 in QApplication::notify(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#6  0x00007ffff1e5f8d8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#7  0x00007ffff1e6204d in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#8  0x00007ffff1eb9263 in  () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007fffec081417 in g_main_context_dispatch () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#10 0x00007fffec081650 in  () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#11 0x00007fffec0816dc in g_main_context_iteration () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#12 0x00007ffff1eb888f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#13 0x00007ffff1e5d90a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#14 0x00007ffff1e669b4 in QCoreApplication::exec() () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
---Type <return> to continue, or q <return> to quit---
#15 0x0000555555572d6a in main(int, char**) (argc=<optimized out>, argv=<optimized out>)
    at ./shell/main.cpp:166

*** This bug has been marked as a duplicate of bug 326289 ***