Вы можете использовать addOnCompleteListener: он отлично работает.
FirebaseInstanceId.getInstance().getInstanceId().addOnCompleteListener(new OnCompleteListener<InstanceIdResult>() {
@Override
public void onComplete(@NonNull Task<InstanceIdResult> task) {
if(!task.isSuccessful()){
Log.w("newToken", "getInstanceId failed", task.getException());
}else{
String deviceToken = task.getResult().getToken();
Log.e("newToken",deviceToken);
RootRef.child("Users").child(currentUserId).child("device_token")
.setValue(deviceToken);
}
}
});