add new value from data model to already exist json file

Some times i need to add new value to my data model, but because i use json as a database, i need to change already exist json files, and add new value in them

error happened when i added new value to CheckList Point class, and then try to display data that was created before i add new value

this is function where i load data from my json files

  static Future<void> loadData() async {
    final dir = await getApplicationDocumentsDirectory();
    List<FileSystemEntity> files = await dir.list().toList();
    for (int i = 0; i < files.length; i++) {
      String filepath = files[i].path;
      File newFile = File(filepath);
      String name = p.basenameWithoutExtension(newFile.path);
      String myExtension = p.extension(filepath);
      if (myExtension != '.cl') {
      } else {
        String data = await newFile.readAsString();
        try {
          checkLists.add(CheckListModel.fromJson(jsonDecode(data)));
        } catch (e) {
          if (e.toString() == "type 'Null' is not a subtype of type 'String' in type cast") {

          }
        }
      }
    }
  }

I have to do something in try cath block, but i have no idea what i can to

also checkLists is static, i use it in listview.builder on another screen of my app

I use json_serializable package to create from json and to json functions

this is functions that i use to write and read data

  Future<File> writeToFile(map) async {
    final file = await localFile;
    return file.writeAsString(jsonEncode(map));
  }

  Future<String> get localPath async {
    final directory = await getApplicationDocumentsDirectory();
    return directory.path;
  }

  Future<File> get localFile async {
    final path = await localPath;
    return File('$path/${_guid}.cl');
  }

CheckListModel class

class CheckListModel {
  static List<CheckListModel> checkLists = [];

  String? _guid;
  String name = "";
  int position = 0;
  String buttonText = "Начать проверку";
  String description = '';
  List<CheckListPoint> checkListPoints = [];

  int checkListCount = 0;
  int chekListExecution = 0;
  int checkListResult = 0;
CheckListModel(this.name, this.position, this.buttonText, this.checkListCount, this.checkListResult, this.chekListExecution, this.description, this.checkListPoints, [var guid]) {
    if (guid == null) {
      Uuid uuid = Uuid();
      _guid = uuid.v1();
    } else {
      _guid = guid.toString();
    }
  }
  get guid {
    return _guid.toString();
  }

checkListPoint class

class CheckListPoint {
  bool correctly = true;
  bool passed = false;
  String requirement = '';
  String notes = " ";
  CheckListPoint(this.correctly, this.passed, this.requirement, this.notes);

  factory CheckListPoint.fromJson(Map<String, dynamic> json) => _$CheckListPointFromJson(json);
  Map<String, dynamic> toJson() => _$CheckListPointToJson(this);
}