"force the move" of users with no room just fails
When a new user is created, and no room is entered in the form (For instance because this specific user wants to support the organization by becoming a member, or because the JavaScript didn't catch the room, for the user didn't "click" on his/her room (this is another issue)) you can still try to force the move. When you do that, the queryset fails when there is more than one member (or former member) without any room.
Proposition of Fix : Check if there is a room which is to be forced empty before querrysetting.