GNOME Shell crashes when deleting workspaces
I could reproduce this using GNOME Tweak Tool: add one ws (-> 5), delete one (-> 4); so far so good. Deleting another one (-> 3) results in the “oh no! something has gone wrong” screen, and the Journal reads:
mutter:ERROR:core/screen.c:1250:update_num_workspaces: assertion failed: (w->windows == NULL)
This looks like https://bugs.debian.org/782075.