使用Firebase InstanceID和RxJava 2.0时出现异常(MAIN_THREAD)

import com.google.firebase.iid.FirebaseInstanceId;
import io.reactivex.Observable;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.schedulers.Schedulers;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 获取Firebase InstanceID令牌
        Observable<String> instanceIdObservable = Observable.fromCallable(() -> FirebaseInstanceId.getInstance().getToken());

        // 在IO线程上执行获取令牌的操作
        instanceIdObservable
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread()) // 将结果切换回主线程
                .subscribe(
                        token -> {
                            // 处理令牌
                            Log.d("FirebaseInstanceId", "Token: " + token);
                        },
                        throwable -> {
                            // 处理异常
                            Log.e("FirebaseInstanceId", "Error getting token", throwable);
                        }
                );
    }
}