Do not leave empty toplevel widgets after closing all open files with "Close All": - it was the left corner workingset indicator widget which was reparented to 0 and stayed visible - the current fix is to hide it before reparenting and show right after adding to the corner - this fix is of course bad, but it's not worse than the code which manages this widget (in general, workingsets do not properly integrate into the UI)

BUG:198251