我需要从MainActivity取消在服务类中启动的计时器，但得到空引用异常

public class MyService extends Service {
    private Timer timer;

    @Override
    public void onCreate() {
        super.onCreate();
        timer = new Timer();
        // 其他初始化操作
    }

    // 启动计时器
    public void startTimer() {
        // 启动计时器逻辑
    }

    // 取消计时器
    public void cancelTimer() {
        if (timer != null) {
            timer.cancel();
            timer = null;
        }
    }
}

public class MainActivity extends AppCompatActivity {
    private MyService myService;
    
    // 绑定服务
    private ServiceConnection serviceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            MyService.MyBinder binder = (MyService.MyBinder) iBinder;
            myService = binder.getService();
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {
            myService = null;
        }
    };

    // 取消计时器
    private void cancelTimer() {
        if (myService != null) {
            myService.cancelTimer();
        }
    }

    // 其他代码
}