如何在nativescript中创建用户后获取fcm的令牌ID（设备ID）？

Question

我在创建用户到firebase之后需要token-id但是代码不能正常工作

//here i am registering to firebase
onRegisterTap() {
  console.log("firebase1");
  firebase.createUser({
    email: this.userRegister.email,
    password: this.userRegister.password
  }).then(
    (result) => {

      // this callback function is not trigering..help me please and i am not gettting token id

      firebase.addOnPushTokenReceivedCallback((token) => {
        console.log("Firebase push token: " + token);
        this.firebaseToken = token;
        console.log("firebase", JSON.stringify(this.firebaseToken));
      }
      );

      sending verification mail
      firebase.sendEmailVerification().then(
        function () {

          console.log("Email verification sent");
        },
        function (error) {
          console.log("Error sending email verification: " + error);
        }
      );
      console.log("firebase", result.key);
    },
    (errorMessage) => {
      this.globals.setIsLoading(false);
      alert({
        title: "Registartion Failed",
        message: errorMessage,
        okButtonText: "OK"
      })
    }
    );
}

//why firebase.addOnPushTokenReceivedCallback method is not triggering?