docs: def on_view_not_found(
              self, environ: Dict[str, Any],
              start_response: Callable[[str, List[Tuple[str, str]]], None],
      ) -> Iterable[bytes]:
          """ called when action is not found """
          start_response(
              "404 Not Found",
              [('Content-type', 'text/plain')])
          return [b"Not Found ", application_uri(environ).encode('utf-8')]