scrollListenerCallback method Null safety

Implementation

void scrollListenerCallback() {
  scrollState.maxOffsetX = scrollController.position.maxScrollExtent;
  scrollState.offsetX = scrollController.offset;

  if (!isScrollReachEnd && scrollState.maxOffsetX == scrollState.offsetX) {
    isScrollReachEnd = true;
    if (widget.onScrollReachEnd != null) widget.onScrollReachEnd!();
    scrollReachEndTimer = Timer(Duration(seconds: 1), () {
      isScrollReachEnd = false;
      scrollReachEndTimer?.cancel();
    });
  } else if (!isScrollReachStart && scrollState.offsetX == 0) {
    isScrollReachStart = true;
    if (widget.onScrollReachStart != null) widget.onScrollReachStart!();
    scrollReachStartTimer = Timer(Duration(seconds: 1), () {
      isScrollReachStart = false;
      scrollReachStartTimer?.cancel();
    });
  }
}