toJson method Null safety

Implementation

@override
Map<String, dynamic> toJson() {
  final ret = super.toJson();
  if (ret['mentioned_user_ids'] == null) {
    List? result = ret['mentioned_users'];

    var userList = result?.map((e) => e['user_id'].toString()).toList();

    if (userList?.isNotEmpty ?? false) {
      ret['mentioned_user_ids'] = userList;
    }
  }
  ret['url'] = uploadFile.url;
  ret['name'] = uploadFile.name;
  ret['size'] = uploadFile.fileSize;
  ret['type'] = uploadFile.mimeType;
  ret.removeWhere((key, value) => value == null);
  return ret;
}