getCategory function Null safety

Implementation

Future<FruitCategory?> getCategory(Iterable<Fruit> path) async {
  if (path.isEmpty) {
    return null;
  }
  FruitCategory? result = categories.value == path.first ? categories : null;
  final List<Fruit> followPath = path.skip(1).toList();
  while (result != null && followPath.isNotEmpty) {
    result = await result.getChild(followPath.first);
    followPath.removeAt(0);
  }
  return result;
}