Every so often I encounter this pop-up when trying to delete a class, and have never really understood the context of why it sometimes appears and sometimes doesn't.     The message seems to suggest that the class is being used by a Plug-in Object, but I know this isn't the case because typically it's a class I've made myself and it hasn't been used in any special way.   Today I finally realized the circumstances that cause this pop-up: objects in that class are be