code: protected function createFormInstance(ContainerInterface $container, $name, array $options = null)
      {
          if (!static::CLASS_NAME || !class_exists(static::CLASS_NAME)) {
              throw new \RuntimeException('The class constants "CLASS_NAME" must be non empty and name an existent class.');
          }
  
          $class = static::CLASS_NAME;
  
          return new $class(null, $options);
      }
docs: Create the form/fieldset instance
  
  @param ContainerInterface $container
  @param string             $name
  @param array              $options
  
  @return \Zend\Form\Fieldset
  @throws \RuntimeException