Created attachment 131716 [details]
backtrace coming from crashed kwin

Application: kwin_x11 (5.19.5)

Qt Version: 5.15.1
Frameworks Version: 5.74.0
Operating System: Linux 5.8.9-arch2-1 x86_64
Windowing system: X11
Distribution: Antergos Linux

Affects: compositing, desktop effects, activities

SUMMARY
I work with application in full screen mode (VMWare Horizon Client) and from time to time I switch to other working in background applications, like Firefox. To switch I use feature 'Present Windows - All Desktop' which is assigned to my right top corner of display, so here I chose window to raise.
Unfortunately from time to time (several times per day) kwin crashes. In result I meet couple issues.

First.
I have 3 'Activities' and in result all windows present on these 'Activities' becomes assigned to 'All Activities', so I need to reassign them again to previous ones. No need to say this is annoying activity. I reported this here: Bug 425105

Second.
Some/Almost all desktop effects (I didn't test all) stop work, like for example used to windows switching feature calling: 'Present Windows - All Desktop' or 'Wobbly Windows'.


The crash can be reproduced sometimes.

-- Backtrace:
In result not so much useful backtrace is generated. For details please check attached file.
I don't know if this is Qt5 bug or issue in kwin.