I had this occuring several times, also in previous upgrades, for about a month or so, and sometimes (after a system upgrade) it was ok again, but now it's back: When I hit the K-button (application menu) in the control panel, KDE crashes completely, i.e. I see a black screen, the control panel disappears, and after a short while everything returns. Once it didn't even return, so I could only switch to console mode and shutdown there in order to get back a working system. This isn't good for unsaved work, and I'm worried. It happened now several times. Right now it's every time I hit the K-Button. Wanted shutdowns are only possible via Ctrl-Alt-Del, which is ok, I guess. Regards
Plasma 4 has been unsupported since October 2015.