code: public function handle(ServerRequestInterface $request): ResponseInterface
      {
          $msg = "This is the default request handler. This means no middleware produced a response before hitting it.";
  
          $contents = json_encode([
              'type' => MiddlewareStackExhaustedException::class,
              'message' => $msg,
          ]);
  
          $response = $this->factory
              ->createResponse(404)
              ->withHeader('Content-type', 'application/json');
  
          $response->getBody()->write($contents);
  
          return $response;
      }
docs: Return a notice json response.
  
  @param \Psr\Http\Message\ServerRequestInterface $request
  @return \Psr\Http\Message\ResponseInterface