code: public function onKernelResponse(FilterResponseEvent $event)
      {
          $routeName = $event->getRequest()->attributes->get('_route');
          if ($routeName !== FallbackRouter::ROUTE_NAME) {
              return;
          }
  
          $response = $event->getResponse();
          if (!$response instanceof LegacyResponse) {
              return;
          }
  
          if (!$this->legacyMode && (int) $response->getStatusCode() === Response::HTTP_NOT_FOUND) {
              $moduleResult = $response->getModuleResult();
              $exception = new NotFoundHttpException(
                  isset($moduleResult['errorMessage']) ?
                      $moduleResult['errorMessage'] :
                      'Not Found'
              );
  
              $exception->setOriginalResponse($response);
  
              throw $exception;
          }
      }
docs: Converts the legacy 404 response to proper Symfony exception.
  
  
  @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
  
  @throws \Symfony\Component\HttpKernel\Exception\NotFoundHttpException