called when action is not found

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')]