code: def change_view(self, *args, **kwargs):
          
          kwargs.setdefault("extra_context", {})
          kwargs["extra_context"]["singleton"] = self.model.objects.count() == 1
          response = super(SingletonAdmin, self).change_view(*args, **kwargs)
          return self.handle_save(args[0], response)
docs: If only the singleton instance exists, pass ``True`` for
          ``singleton`` into the template which will use CSS to hide
          the "save and add another" button.