getAdminRestrictedOperationHelpLinkCallback() {
    const adminRestrictedOperation =
        this.config_.get('adminRestrictedOperation') || null;
    if (adminRestrictedOperation) {
      const helpLink = adminRestrictedOperation['helpLink'] || null;
      if (helpLink && typeof helpLink === 'string') {
        return () => {
          util.open(
              /** @type {string} */ (helpLink),
              util.isCordovaInAppBrowserInstalled() ?
              '_system' : '_blank');
        };
      }
    }
    return null;
  }
@return {?function()} The callback to redirect to the help link when users
    are restricted from sign up. Otherwise, null is returned.