SUMMARY
***
After a while (I notice that this happens after switching to another activity), I am unable to switch between applications with keyboard nor with clicking on "Icons only task manager". Activity switching kind of works (icons in task manager change), but actual active window doesn't change too.

NOTE: If you are reporting a crash, please try to attach a backtrace with debug symbols.
See https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports
***


STEPS TO REPRODUCE
1. Happens randomly, I assume, after switching to another activity.

OBSERVED RESULT
Can't switch between applications with neither keyboard nor mouse.

If I check `dmesg`, there is nothing suspiciuos, `journalctl --since today --user --unit=plasma-plasmashell` gives this at:
```
May 17 18:29:03 enmk-xps9570 plasmashell[3961]: SwitcherBackend: FAILED to get the thumbnail "Unknown error code 0\n\nPlease send a full bug report at https://bugs.kde.org." ("Undocumented Error", "<qt><p><b>Undocumented Error</b></p><p>Unknown error code 0\n>
May 17 18:29:03 enmk-xps9570 plasmashell[3961]: SwitcherBackend: FAILED to get the thumbnail "Unknown error code 0\n\nPlease send a full bug report at https://bugs.kde.org." ("Undocumented Error", "<qt><p><b>Undocumented Error</b></p><p>Unknown error code 0\n>
May 17 18:29:03 enmk-xps9570 plasmashell[3961]: qml: PlasmaExtras.ScrollArea is deprecated. Use PlasmaComponents3.ScrollView instead.
May 17 18:29:03 enmk-xps9570 plasmashell[3961]: file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:185: TypeError: Value is null and could not be converted to an object
May 17 18:29:18 enmk-xps9570 plasmashell[3961]: SwitcherBackend: FAILED to get the thumbnail "Unknown error code 0\n\nPlease send a full bug report at https://bugs.kde.org." ("Undocumented Error", "<qt><p><b>Undocumented Error</b></p><p>Unknown error code 0\n>
May 17 18:29:18 enmk-xps9570 plasmashell[3961]: SwitcherBackend: FAILED to get the thumbnail "Unknown error code 0\n\nPlease send a full bug report at https://bugs.kde.org." ("Undocumented Error", "<qt><p><b>Undocumented Error</b></p><p>Unknown error code 0\n>
May 17 18:29:18 enmk-xps9570 plasmashell[3961]: qml: PlasmaExtras.ScrollArea is deprecated. Use PlasmaComponents3.ScrollView instead.
May 17 18:29:19 enmk-xps9570 plasmashell[3961]: file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:185: TypeError: Value is null and could not be converted to an object
May 17 18:29:21 enmk-xps9570 plasmashell[3961]: SwitcherBackend: FAILED to get the thumbnail "Unknown error code 0\n\nPlease send a full bug report at https://bugs.kde.org." ("Undocumented Error", "<qt><p><b>Undocumented Error</b></p><p>Unknown error code 0\n>
May 17 18:29:21 enmk-xps9570 plasmashell[3961]: SwitcherBackend: FAILED to get the thumbnail "Unknown error code 0\n\nPlease send a full bug report at https://bugs.kde.org." ("Undocumented Error", "<qt><p><b>Undocumented Error</b></p><p>Unknown error code 0\n>
May 17 18:29:21 enmk-xps9570 plasmashell[3961]: qml: PlasmaExtras.ScrollArea is deprecated. Use PlasmaComponents3.ScrollView instead.
May 17 18:29:22 enmk-xps9570 plasmashell[3961]: file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:185: TypeError: Value is null and could not be converted to an object
```
If I try to restart plasma with `kquitapp5 plasmashell || killall plasmashell && kstart5 plasmashell` from `krunner` or terminal, it seems to be restarting incorrectly: keyboard input is not working at all, window decorations are missing.
Had to restart OS to fix it.

EXPECTED RESULT
Can switch between applications within activity / switch between activities. OR restarting plasma fixes the issue.

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.27
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.105.0
Qt Version: 5.15.9
Kernel Version: 5.19.0-41-generic (64-bit)
Graphics Platform: X11
Processors: 12 × Intel® Core™ i7-8750H CPU @ 2.20GHz
Memory: 31.0 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 630
Manufacturer: Dell Inc.
Product Name: XPS 15 9570

ADDITIONAL INFORMATION
Started to happen only recently, was working fine before system update. Single built-in single-monitor setup. Maybe related to low available RAM on machine (used by other process excessively), no swap.