I'm trying to create a simple widget which counts down from 10 upon getting built. I expected this to start counting down but it remains stuck on 10. Could anyone see what is going wrong here?

```
class GameTimer extends StatefulWidget {
  const GameTimer({
    Key? key,
  }) : super(key: key);

  @override
  State<GameTimer> createState() => _GameTimerState();
}

class _GameTimerState extends State<GameTimer> {
  initState() {
    _startTimer();
  }

  int _counter = 10;
  late Timer _timer;

  void _startTimer() {
    _counter = 10;
    _timer = Timer.periodic(Duration(seconds: 1), (timer) {
      setState() {
        _counter--;
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return SizedBox(
        child: Text('$_counter',
            style: TextStyle(
              fontSize: 48,
              fontWeight: FontWeight.bold,
            )));
  }
}

```