function getContinueCallback(url = undefined) {
  const continueUrl = utils.getParamValue(
       url || util.getCurrentUrl(), 'continueUrl');
  // If continue URL detected, return a callback URL to redirect to it.
  if (continueUrl) {
    return () => {
      util.goTo(/** @type {string} */ (continueUrl));
    };
  }
  return null;
}
Gets the action code continue callback from the given URL. If no URL is
provided, no continue button is shown. This gives the user the ability to
go back to the application. This could open an FDL link which redirects to a
mobile app or to a web page. If no continue URL is available, no button is
shown.
@param {?string=} url The URL from which to extract the continue URL.
@return {?function()} The continue callback that will redirect the page back
    to the app. If none available, null is returned.