code: private function findHandlerForMessage($messageName)
      {
          $handler = null;
  
          if (array_key_exists($messageName, $this->responseHandlers) &&
              $this->responseHandlers[$messageName] instanceof MessageResponseHandler
          ) {
              $handler = $this->responseHandlers[$messageName];
          } else {
              $section = substr($messageName, 0, strpos($messageName, '_'));
              $message = substr($messageName, strpos($messageName, '_') + 1);
  
              $handlerClass = __NAMESPACE__.'\\'.$section.'\\Handler'.$message;
  
              if (class_exists($handlerClass)) {
                  /** @var MessageResponseHandler $handler */
                  $handler = new $handlerClass();
  
                  $this->responseHandlers[$messageName] = $handler;
              }
          }
  
          return $handler;
      }
docs: Find or create the correct handler object for a given message
  
  @param string $messageName
  @return MessageResponseHandler|null