code: def _has_fr_route(self):
          
          # 404's, 405's, which might not have a url_rule
          if self._should_use_fr_error_handler():
              return True
          # for all other errors, just check if FR dispatched the route
          if not request.url_rule:
              return False
          return self.owns_endpoint(request.url_rule.endpoint)
docs: Encapsulating the rules for whether the request was to a Flask endpoint