InstanceId solution for LadyM (License: https://gist.github.com/liorkup/019cec4f23edfe39a444571206ea2ae7)

MainActivity.Java

//InstanceId solution
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        prefs = getSharedPreferences("com.mycompany.myAppName", MODE_PRIVATE);
        setContentView(R.layout.activity_main);
    
    }


    @Override
    protected void onResume() {
        super.onResume();

        if (prefs.getBoolean("firstrun", true)) {
            // Do first run stuff here then set 'firstrun' as false
            // using the following line to edit/commit prefs
            prefs.edit().putBoolean("firstrun", false).commit();

            @SuppressLint("StaticFieldLeak") AsyncTask<Void, Void, String> task = new AsyncTask<Void, Void, String>() {
                @Override
                protected String doInBackground(Void... params) {
                    AdvertisingIdClient.Info idInfo = null;
                    try {
                        idInfo = AdvertisingIdClient.getAdvertisingIdInfo(getApplicationContext());
                    } catch (GooglePlayServicesNotAvailableException e) {
                        e.printStackTrace();
                    } catch (GooglePlayServicesRepairableException e) {
                        e.printStackTrace();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    String advertId = null;
                    try{
                        storeIdInServer(mFirebaseAnalytics.getFirebaseInstanceId(), idInfo.getId());
                     }catch (NullPointerException e){
                        e.printStackTrace();
                    }

                    return advertId;
                }

                private void storeIdInServer(String firebaseInstanceId, String advertisingId) {
                    //todo
                    System.out.println( String.format("InstanceId: %s; AdvertisingId: %s", mFirebaseAnalytics.getFirebaseInstanceId(), advertisingId));

                }

                @Override
                protected void onPostExecute(String advertId) {
                    Toast.makeText(getApplicationContext(), advertId, Toast.LENGTH_SHORT).show();
                }

            };
            task.execute();
            
        }
    }
}}