code: public function serialize()
      {
          try {
              $this->data = $this->data ?: $this->serializer->getData($this->closure, true);
              return serialize($this->data);
          } catch (\Exception $e) {
              trigger_error(
                  'Serialization of closure failed: ' . $e->getMessage(),
                  E_USER_NOTICE
              );
              // Note: The serialize() method of Serializable must return a string
              // or null and cannot throw exceptions.
              return null;
          }
      }
docs: Serializes the code, context, and binding of the closure.
  
  @return string|null
  @link http://php.net/manual/en/serializable.serialize.php