Android: Firebase token is null at first run

Question

I am using FCM to provide notifications in my app. Everything worked well, but now I realised that, when I install my application using Android Studio (not from GooglePlay) the token is null at first run. When I close my app and restart it, the token is not null anymore. What cause this problem and how can I avoid it?

InstanceIDService:

public class InstanceIDService extends FirebaseInstanceIdService {

    @Override
    public void onTokenRefresh() {

        String token = FirebaseInstanceId.getInstance().getToken();
        registerToken(token);
    }

    private void registerToken(String token) {

        OkHttpClient client = new OkHttpClient();
        RequestBody body = new FormBody.Builder()
                .add("Token",token)
                .build();

        Request request = new Request.Builder()
                .url("url_to_registration_script")
                .post(body)
                .build();

        try {
            client.newCall(request).execute();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In MainActivity:

FirebaseMessaging.getInstance().subscribeToTopic("topic");
String token = FirebaseInstanceId.getInstance().getToken();
Log.d("TOKEN", token);

Last log returns null when app is installed and started for the first time

Registration script:

 <?php 
 if (isset($_POST["Token"])) {

       $_uv_Token=$_POST["Token"];
       $conn = mysqli_connect("localhost","user","pass","db") or die("Error connecting");
       $q="INSERT INTO users (Token) VALUES ( '$_uv_Token') "
          ." ON DUPLICATE KEY UPDATE Token = '$_uv_Token';";

  mysqli_query($conn,$q) or die(mysqli_error($conn));
  mysqli_close($conn);
}
?>

Answer 1

The token is fetched asynchronously on first app start. You have to wait for onTokenRefresh() to be called in your FirebaseInstanceIdService before the token can be accessed.