code: public function onException(GetResponseForExceptionEvent $event)
      {
          if (!$this->isAdminSiteAccess) {
              return;
          }
  
          $routeName = $event->getRequest()->attributes->get('_route');
          if ($routeName !== FallbackRouter::ROUTE_NAME) {
              return;
          }
  
          $exception = $event->getException();
          if (!$exception instanceof NotFoundHttpException) {
              return;
          }
  
          // We're supporting admin UI exception as well as Netgen
          // internal exception
          if (method_exists($exception, 'getOriginalResponse')) {
              $event->setResponse($exception->getOriginalResponse());
          }
      }
docs: Handles the legacy exceptions.
  
  @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event