/**
  * {@inheritdoc}
  */
 public function buildExposeForm(&$form, FormStateInterface $form_state)
 {
     parent::buildExposeForm($form, $form_state);
     if ($this->options['type'] != 'select') {
         unset($form['expose']['reduce']);
     }
     $form['error_message'] = array('#type' => 'checkbox', '#title' => $this->t('Display error message'), '#description' => $this->t('Display an error message if one of the entered terms could not be found.'), '#default_value' => !empty($this->options['error_message']));
 }