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