Have a Warning when Trying to Delete Group Containing Objects which have Occurrence(s)

When deleting a group which has any type of objects, there is a general warning that the group is not empty. Since the group view defaults to just models, it's possible that the user may not be aware that the group has objects that are still used by models within it. A warning should be made to tell the user that the group that will be deleted contains objects that are being used by model(s).