called when views 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 views not found"""
        start_response('404 Not Found', [('Content-type', 'text/plain')])
        return [b'Not found']