[SOLVED] Null is not a subtype

I have 10 attributes in my collection and when I run the application it gives an error at this point:

    databases ?? init();
    try {
      String userId = await AuthHelper.instance.getUserId() ?? "";
      DocumentList response = await databases!.listDocuments(
        databaseId: "64046c8b50eba8c807d0",
        collectionId: "6405162fd68c7c61df67",
      );
      return response.documents
          .map(
            (e) => TodoModel.fromJson(e.data, e.$id,)
          )
          .toList();
    } catch (e) {
      rethrow;
    }
  }```

Exception has occurred.
_TypeError (type 'Null' is not a subtype of type 'String')

This is what my data model looks like:

class TodoModel {
String name;
int scores;
double progress;
List errors;
List friends;
int alchiki;
String refcode;
DateTime createdAt;
String id;
String userId;

TodoModel({
required this.name,
required this.scores,
required this.progress,
required this.errors,
required this.friends,
required this.alchiki,
required this.refcode,
required this.createdAt,
required this.userId,
required this.id,
});

TodoModel.fromJson(Map<String, dynamic> json, String tempId)
: name = json['first_name'],
scores = json['scores'],
progress = json['progress'],
errors = json['errors'],
friends = json['friends'],
alchiki = json['alchiki'],
refcode = json['refcode'],
createdAt = DateTime.parse(json['createdAt']),
userId = json['userId'],
id = tempId;

Map<String, dynamic> toJson() => {
'first_name': name,
'scores': scores,
'progress': progress,
'errors': errors,
'friends': friends,
'alchiki': alchiki,
'refcode': refcode,
'createdAt': createdAt.toIso8601String(),
'userId': userId,
};
}

If I remove the "refcode" attribute from the model, then the application will work fine.

it seems you have a document where refcode is null, right?

SOLVED

[SOLVED] Null is not a subtype