For this bug, you need a background in the "Places" menu and use this background in a message in the "Storyboard" with the action "Ask player to point an area".
When you want to delete a place, a warning message appears (example: "Warning! This place is used in message 451, 523" etc.). However, if the place is only used with the "Ask player to point an area" action and not as a background in the "Screen editor", this warning message doesn't appear and you can delete the background without any problem. The problem is that after deleting the place, in messages where the background was used with the action "Ask player to point an area ", the button "Set action" doesn't work anymore.
I say "minor bug" because just thinking about changing the action before deleting the background is enough and the trial is still intact, but inattentions arrive quickly.