Issue #3172576 by AdamPS, thomasmurphy, morvaim, bsuttis: Admin user cannot...

   */

  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {

    $form = parent::buildConfigurationForm($form, $form_state);

    $allowed = \Drupal::service('administerusersbyrole.access')->listRoles('role-assign', \Drupal::currentUser());

    $account = \Drupal::currentUser();

    if (!$account->hasPermission('administer users')) {

      $allowed = \Drupal::service('administerusersbyrole.access')->listRoles('role-assign', $account);

      $form['rid']['#options'] = array_intersect_key($form['rid']['#options'], array_flip($allowed));

    }

    return $form;

  }