type null is not a subtype of type 'String' error

Question

 body: StreamBuilder<List<User>>(
        stream: readUsers(),
        builder: (context, snapshot) {
          if (snapshot.hasError) {
            return Text('Error ${snapshot.error}');
          }
          if (snapshot.hasData) {
            final users = snapshot.data!;

            return ListView(
              children: users.map(buildUser).toList(),
            );
          } else {
            return Center(
              child: CircularProgressIndicator(),
            );
          }
        },
      )),

The part where I am trying to pull data from firebase.

  Widget buildUser(User user) => ListTile(leading: Text('${user.bmi}'));

The buildUser widget I created.

  Stream<List<User>> readUsers() => FirebaseFirestore.instance
  .collection('users')
  .snapshots()
  .map((snapshot) =>
      snapshot.docs.map((doc) => User.fromJson(doc.data())).toList());

The readUser structure I created

I am able to print data to the database and one of these data is a double value named 'bmi'. When I continue the process, it stays in the snapshot.haserror loop and

type null is not a subtype of type 'String'

prints the error.

    class User {
  final double bmi;

  User(
      {
      required this.bmi});

  Map<String, dynamic> toJson() => {
        'bmi': bmi,
      };

  static User fromJson(Map<String, dynamic> json) => User(
      bmi: json['bmi']);
}

I am getting some values on the previous page from the user. Then I go to the page with the error with a button. It also prints the error I asked in the question inside the page. — ckot
– ckot, Commented Dec 6, 2022 at 16:15
maybe your error come from another model , in this class your provided there is no string type at all — Sayyid J
– Sayyid J, Commented Dec 6, 2022 at 16:26

rszf · Accepted Answer · 2022-12-06 16:12:21Z

2

i am not 100% sure, but i had the same error, where the json did not have that specific field. Before you try to get the 'bmi' field of json, check if it exists:

json.containsKey("bmi") ? json['bmi'] : "missing field"

answered Dec 6, 2022 at 16:12

rszf

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1 Comment

ckot Over a year ago

Unfortunately nothing has changed.

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