Display extenders are now validated when the view is saved

Now when you save a view, display extenders are validated like other views plugins, handlers, relationships, etc. You can add your own validation using the validate() method of your display extender plugin.

Example:

/**
 * {@inheritdoc}
 */
public function validate() {
  $errors = parent::validate();

  // Check the empty value.
  if (empty($this->options['not_empty_value'])) {
    $errors[] = $this->t('The value is empty.');
  }
  return $errors;
}