serialize method Null safety

Implementation

static Map<String, Object?> serialize(
  PlatformMenu item,
  PlatformMenuDelegate delegate,
  MenuItemSerializableIdGenerator getId,
) {
  final List<Map<String, Object?>> result = <Map<String, Object?>>[];
  for (final MenuItem childItem in item.menus) {
    result.addAll(childItem.toChannelRepresentation(
      delegate,
      getId: getId,
    ));
  }
  // To avoid doing type checking for groups, just filter out when there are
  // multiple sequential dividers, or when they are first or last, since
  // groups may be interleaved with non-groups, and non-groups may also add
  // dividers.
  Map<String, Object?>? previousItem;
  result.removeWhere((Map<String, Object?> item) {
    if (previousItem == null && item[_kIsDividerKey] == true) {
      // Strip any leading dividers.
      return true;
    }
    if (previousItem != null && previousItem![_kIsDividerKey] == true && item[_kIsDividerKey] == true) {
      // Strip any duplicate dividers.
      return true;
    }
    previousItem = item;
    return false;
  });
  if (result.isNotEmpty && result.last[_kIsDividerKey] == true) {
    result.removeLast();
  }
  return <String, Object?>{
    _kIdKey: getId(item),
    _kLabelKey: item.label,
    _kEnabledKey: item.menus.isNotEmpty,
    _kChildrenKey: result,
  };
}