code: public function handle($message, callable $next)
      {
          try {
              $next($message);
          } catch (UndefinedCallable $exception) {
              $this->logger->log(
                  $this->logLevel,
                  'No message handler found, trying to handle it asynchronously',
                  [
                      'type' => get_class($message)
                  ]
              );
  
              $this->publisher->publish($message);
          }
      }
docs: Handle the message by letting the next middleware handle it. If no handler is defined for this message, then
  it is published to be processed asynchronously
  
  @param object $message
  @param callable $next