main.dart

@override
void initState() {
  super.initState();
  animationController = AnimationController(vsync: this, duration: Duration(milliseconds: 300));
  animationController.addStatusListener((status) {
    if (status == AnimationStatus.completed && isPlaying) { //<-- Add this check
      if (notes[currentNoteIndex].state != NoteState.tapped) { //<--Add this check
        //game over
        setState(() {
          isPlaying = false; //<--Stop the game
        });
      } else if (currentNoteIndex == notes.length - 5) {
        //song finished
      } else {
        setState(() => ++currentNoteIndex);
        animationController.forward(from: 0);
      }
    }
  });
}