Ionic-native firebase getToken()

ionic-native firebase getToken()
I don’t seem to get any response from getToken at all?
I even tried triggering it manually with a button.

...
import { Firebase } from '@ionic-native/firebase/ngx';
...
constructor(public firebaseNative: Firebase),
...
async getToken() {
let token;
token = await this.firebaseNative.getToken();
alert(token);
}

I get nothing back, not even an error?

successfully Installed: ionic cordova plugin add cordova-plugin-firebase
successfully Installed: npm install @ionic-native/firebase
I compiled the app on using ionic pro / app flow and installed it on a real Android device, no emulator.

What am I doing wrong please?
How can I retrieve any error messages that may be being returned by getToken() ?