Issue #3269512 by Luke.Leber, carolpettirossi: Fatal error when creating an...

    return $form;

  }

  /**

   * {@inheritdoc}

   */

  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {

    parent::validateConfigurationForm($form, $form_state);

    // Ensure that the allowed_ids sequence is stored without keys.

    $value_path = ['settings', 'handler_settings', 'filter', 'allowed_ids'];

    $with_keys = $form_state->getValue($value_path);

    $without_keys = array_values($with_keys);

    $form_state->setValue($value_path, $without_keys);

  }

}