code: function(resp, opt_error) {
      if (resp && resp['account'] && opt_onAccountSelected) {
        opt_onAccountSelected(
            firebaseui.auth.Account.fromPlainObject(resp['account']));
      } else if (opt_onAddAccount) {
        // Check if accountchooser.com is available and pass to add account.
        var isUnavailable = firebaseui.auth.acClient.isUnavailable_(opt_error);
        // Either an error happened or user clicked the add account button.
        opt_onAddAccount(!isUnavailable);
      }
    }
docs: Save the add account callback for later use.