close method Null safety
Implementation
Future<void> close() async {
  if (!_isOpen) {
    return;
  }
  flush();
  _isOpen = false;
  await _fp.close();
}Future<void> close() async {
  if (!_isOpen) {
    return;
  }
  flush();
  _isOpen = false;
  await _fp.close();
}