public function logout($campaign_id = NULL) {
    $response = new AjaxResponse();

    // Logout member - clear Campaign ID from SESSION.
    MemberCampaign::logout($campaign_id);

    $logoutTitle = $this->t('Thank you!');
    $logoutMessage = $this->t('You were successfully logged out!');

    $response->addCommand(new OpenModalDialogCommand($logoutTitle, $logoutMessage, ['width' => 800]));

    // Close dialog and redirect ot Campaign main page.
    $response->addCommand(new InvokeCommand('#drupal-modal', 'closeDialog', ['<campaign-front>']));

    return $response;
  }