Application terminates when mouse is moved over busy (blocked) GUI

If the application GUI thread is temporarily blocked by a time-consuming work and mouse is moved over it -> application crashes as soon as the thread gets unblocked. There is no backtrace to this, it ends with error

The Wayland connection broke. Did the Wayland compositor die?

It can be reproduced e.g. with Krusader:

1. Start Krusader
2. Click on "MountMan" toolbar button
3. Immediately after that start moving your mouse over Krusader app.
4. Krusader terminates

Try it with no mouse movement after the click and all should be fine.

This was reproduced on Plasma (git master) as well as Weston 3.0.0.

I'll provide a minimal example app. on demand.