Convert a json string to list of objects in flutter

Question

Im trying to convert a json string to a list of objects in my flutter project. When I try to do that I get this error

Unhandled Exception: NoSuchMethodError: Class 'String' has no instance method 'map'.

My Object looks like this

class UpdateItem {
  final String? title;
  final bool? selected;

  UpdateItem({this.title, this.selected});

  factory UpdateItem.fromJson(dynamic json) {
    return UpdateItem(
      title: json['title'] as String,
      selected: json['selected'] as bool,
    );
  }

  @override
  String toString() {
    return '{ ${this.title}, ${this.selected} }';
  }

  Map toJson() => {
        'title': title,
        'selected': selected,
      };
}

where I set list of objects to json and save that in shared preferences.

Future<String?> getSelectedList(String k) async =>
      preferenceHelper.getStringPrefs(k);

  setSelectedList(String k, dynamic d) async =>
      preferenceHelper.updateJsonPrefs(k, d);

updateStringPrefs(String key, String value) async {
    prefs = await SharedPreferences.getInstance();
    prefs.setString(key, value);
  }

Future<String?> getStringPrefs(String key) async {
    prefs = await SharedPreferences.getInstance();
    return prefs.getString(key) ?? "";
  }


Future<void> saveSelectedList(List<UpdateItem> list) async {
    var title = _categoryModelList![pageNo].title;
    String encodedData = json.encode(list);
    DataManager _dataManager = new DataManager();
    _dataManager.setSelectedList(title!, encodedData);
    //
    _updateItemProvider!.removeAll();
  }

Where I try to get that saved json string and convert it to a List of objects.

for (var i = 0; i < _categoryModelList!.length; i++) {
      String s = _categoryModelList![i].title!;
      var t = await _dataManager.getSelectedList(s);
      var updateObj = json.decode(t!);
      
// error referee here.......
List<UpdateItem> list =
          updateObj.map((e) => UpdateItem.fromJson(e)).toList();
      print(list);
    }

Where and what i'm doing wrong here. Can anyone help me out with this please? Here is the json response I get

\"[{\\"title\\":\\"Hearing Loops\\",\\"selected\\":true},{\\"title\\":\\"Hearing Assistance\\",\\"selected\\":true}]\"