Reproducibility: Often. Same happens when pressing on 'Add Widgets...' button or pressing one more time on Panel settings button. Also after a number of Plasma restarts because of this issue - Plasma is not starting - displaying only a black screen.
For crash reports, can you please try and use the crash reporting assistant. Without a trace they're hard to match up duplicates or do anything about them. *** This bug has been marked as a duplicate of bug 381630 ***