getEmailProviderHelpLinkCallBack() {
    const emailSignInOption = this.getSignInOptionsForProvider_(
        firebase.auth.EmailAuthProvider.PROVIDER_ID);
    if (emailSignInOption && emailSignInOption['disableSignUp']) {
      const helpLink = emailSignInOption['disableSignUp']['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 email
    provider sign up is disabled.