validateExtraOptionsForm example

      $view->getStandardButtons($form, $form_state, 'views_ui_config_item_extra_form');
    }
    return $form;
  }

  /** * {@inheritdoc} */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $form_state->get('handler')->validateExtraOptionsForm($form['options'], $form_state);
  }

  /** * {@inheritdoc} */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $view = $form_state->get('view');
    $handler = $form_state->get('handler');
    // Run it through the handler's submit function.     $handler->submitExtraOptionsForm($form['options'], $form_state);
    $item = $handler->options;