LateInitializationError: Field '_timer@137128430' has not been initialized in flutter Null safe version

I'm beginner for the flutter , I have added flowing code and flutter suggested to add late, I added late Timer _timer; after it showed me the following error

LateInitializationError: Field '_timer@137128430' has not been initialized.

any solution for this? Thanks

code here

  late Timer _timer;
  int _start = 60;

  //start timer
  void startTimer() {
    const oneSec = const Duration(seconds: 1);
    _timer = new Timer.periodic(
      oneSec,
          (Timer timer) {
        if (_start == 0) {
          setState(() {
            timer.cancel();
          });
        } else {
          setState(() {
            _start--;
          });
        }
      },
    );
  }
  @override
  void initState() {
    super.initState();
    startTimer();
    
  }

  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
  }