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.

def change_view(self, *args, **kwargs):
        """
        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.
        """
        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)