更新MutableLiveData时观察到的抛出异常

尝试更新MutableLiveData时遇到问题。我正在从ViewModel调用登录功能以更新我的UI。在ViewModel中，我对API服务器进行了调用，但是当我调用.notify（）时，发生了崩溃：

java.lang.IllegalMonitorStateException：对象在notify（）之前没有被线程锁定

我应该在哪里放置它以便工作

这是我的XML文件

    <data>
        <variable
            name="viewModel"
            type="com.kidzmedia.radio.activities.login.LoginViewModel" />
    </data>

....

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="96dp"
        android:onClick="@{() -> viewModel.doLogin()}"
        android:text="Login" />

我的ViewModel

public class LoginViewModel extends ViewModel {
    private MutableLiveData<User> userMutableLiveData;

....

    public void doLogin() {
        userMutableLiveData = userApi.loginUser(email, password);
        Log.i("LOGIN", "do Login");

        //userMutableLiveData.setValue(tmpUser.getValue());
    }

    LiveData<User> getUser() {
        if (userMutableLiveData == null) {
            userMutableLiveData = new MutableLiveData<>();
        }

        return userMutableLiveData;
    }

我的UI

public class LoginActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ActivityLoginBinding activityLoginBinding = DataBindingUtil.setContentView(this, R.layout.activity_login);
        //activityLoginBinding.setViewModel(ViewModelProviders.of(this).get(LoginViewModel.class);

        LoginViewModel loginViewModel = ViewModelProviders.of(this).get(LoginViewModel.class);
        activityLoginBinding.setViewModel(loginViewModel);

        loginViewModel.getUser().observe(this, new Observer<User>() {

            @Override
            public void onChanged(User user) {
                if (user != null)
                    // DO STUFF
            }
        });
    }
}

我的Api服务器：

public class UserApi {

    private final String TAG = getClass().getSimpleName();

    public MutableLiveData<User> loginUser(String email, String password) {


        User loginUser = new User(email, password);

        final MutableLiveData<User> mutableLiveData = new MutableLiveData<>();

        UserService userApi = APIUtils.getUserService();
        userApi.doLoginUser(loginUser).enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, Response<User> response) {
                mutableLiveData.setValue(response.body());

                // THIS IS WHERE IT CRASHES
                mutableLiveData.notify();
            }
        });

        return mutableLiveData;
    }