Setup:
 - kubuntu 17.10;
 - plasma v5.11.5;
 - QT: 5.9.1;
 - video card NVIDIA GTX 1080Ti;
 - video driver: nouveau (v1.0.15-2);
 - primary monitor: BENQ connected through DisplayPort;
 - secondary monitor (smaller resolution): LG connected through HDMI->VGA converter;
 - secondary monitor is on the left of primary monitor;

Problem:
 - if I turn off primary monitor when logged in then KDE crashes and login screen appears;
 - if I turn on primary monitor after log on then KDE crashes and login screen appears;

After KDE crash I still can see picture (login screen) on secondary monitor.

Entries from kernel log (several crashes after I played with power button of primary monitor):

2018-01-19 10:21	usb 1-6	USB disconnect, device number 2
2018-01-19 10:21	nouveau 0000	6:00.0: disp: 0x00006541[0]: INIT_GENERIC_CONDITON: unknown 0x07
2018-01-19 10:21	show_signal_msg	10 callbacks suppressed
2018-01-19 10:21		kscreen_backend[1444]: segfault at 10 ip 00007ff3a9466968 sp 00007ffe1faae640 error 4 in KSC_XRandR.so[7ff3a944e000+21000]
2018-01-19 10:21	nouveau 0000	6:00.0: gr: FECS falcon already acquired by gr!
2018-01-19 10:21	nouveau 0000	6:00.0: gr: init failed, -16
2018-01-19 10:21	usb 1-6	new high-speed USB device number 3 using xhci_hcd
2018-01-19 10:21	usb 1-6	New USB device found, idVendor=05e3, idProduct=0608
2018-01-19 10:21	usb 1-6	New USB device strings: Mfr=0, Product=1, SerialNumber=0
2018-01-19 10:21	usb 1-6	Product: USB2.0 Hub
2018-01-19 10:21	hub 1-6	.0: USB hub found
2018-01-19 10:21	hub 1-6	.0: 4 ports detected
2018-01-19 10:22		kscreen_backend[3140]: segfault at 14 ip 00007f207d87ebb2 sp 00007ffd44f5efa0 error 4 in KSC_XRandR.so[7f207d86a000+21000]
2018-01-19 10:22	nouveau 0000	6:00.0: gr: FECS falcon already acquired by gr!
2018-01-19 10:22	nouveau 0000	6:00.0: gr: init failed, -16
2018-01-19 10:22	usb 1-6	USB disconnect, device number 3
2018-01-19 10:22	nouveau 0000	6:00.0: disp: 0x00006541[0]: INIT_GENERIC_CONDITON: unknown 0x07
2018-01-19 10:22		kscreen_backend[3455]: segfault at 10 ip 00007fb6d7df6968 sp 00007ffd1dd2c7c0 error 4 in KSC_XRandR.so[7fb6d7dde000+21000]
2018-01-19 10:22	nouveau 0000	6:00.0: gr: FECS falcon already acquired by gr!
2018-01-19 10:22	nouveau 0000	6:00.0: gr: init failed, -16
2018-01-19 10:23	usb 1-6	new high-speed USB device number 4 using xhci_hcd
2018-01-19 10:23	usb 1-6	New USB device found, idVendor=05e3, idProduct=0608
2018-01-19 10:23	usb 1-6	New USB device strings: Mfr=0, Product=1, SerialNumber=0
2018-01-19 10:23	usb 1-6	Product: USB2.0 Hub
2018-01-19 10:23	hub 1-6	.0: USB hub found
2018-01-19 10:23	hub 1-6	.0: 4 ports detected
2018-01-19 10:23		kscreen_backend[4005]: segfault at 14 ip 00007f9292aafbb2 sp 00007fffa169f410 error 4 in KSC_XRandR.so[7f9292a9b000+21000]
2018-01-19 10:23	nouveau 0000	6:00.0: gr: FECS falcon already acquired by gr!
2018-01-19 10:23	nouveau 0000	6:00.0: gr: init failed, -16
2018-01-19 10:23	nouveau 0000	6:00.0: gr: init failed, -16