Flutter: I'm getting the type 'Null' is not a subtype of type 'String' when I am using StreamBuilder

Question

Hello guys I am using a stream builder to get data from fireStore collection to display some posts but I am getting an error saying type 'Null' is not a subtype of type 'String' and the value of the snapshot is null

this is the post model that I have

class Post {


 final String id;
  final String description;
  final String validUntil;
  final String availableFor;
  final String imageUrl;
  final String owner;



Post({
    required this.id,
    required this.description,
    required this.validUntil,
    required this.availableFor,
    required this.imageUrl,
    required this.owner,
  });

  Map<String, dynamic> toJson() => {
        'id': id,
        'description': description,
        'validUntil': validUntil,
        'availableFor': availableFor,
        'imageUrl': imageUrl,
        'owner': owner,
      };

  static Post fromJson(Map<String, dynamic> json) => Post(
        id: json['id'],
        description: json['description'],
        validUntil: json['validUntil'],
        availableFor: json['availableFor'],
        imageUrl: json['imageUrl'],
        owner: json['owner'],
      );
}

and this is the Stream

Stream<List<Post>> readPosts() => FirebaseFirestore.instance
  .collection('posts')
  .snapshots()
  .map((snapshot) =>
      snapshot.docs.map((doc) => Post.fromJson(doc.data())).toList());

and this is the StreamBuilder

StreamBuilder<List<Post>>(
                stream: readPosts(),
                builder: (context, snapshot) {
                  if (snapshot.connectionState == ConnectionState.active) {
                    if (snapshot.data == null) {
                      print(snapshot.error.toString());
                      return Text(snapshot.error.toString());
                    } else {
                      final posts = snapshot.data;
                      return Column(
                        children: posts!.map(buildPost).toList(),
                      );
                    }
                  } else {
                    return Center(
                      child: CircularProgressIndicator(),
                    );
                  }
                })

I would really appreciate it if you can help me solve this problem

Answer 1

You have some data as a null value. To fix it you can default any null value to something else with the null coalescing operator ??.

The code's going to look like this:

class Post {
  final String id;
  final String description;
  final String validUntil;
  final String availableFor;
  final String imageUrl;
  final String owner;

  Post({
    required this.id,
    required this.description,
    required this.validUntil,
    required this.availableFor,
    required this.imageUrl,
    required this.owner,
  });

  Map<String, dynamic> toJson() => {
        'id': id,
        'description': description,
        'validUntil': validUntil,
        'availableFor': availableFor,
        'imageUrl': imageUrl,
        'owner': owner,
      };

  Post.fromJson(Map<String, dynamic> json)
      : id = json['id'] ?? '',
        description = json['description'] ?? '',
        validUntil = json['validUntil'] ?? '',
        availableFor = json['availableFor'] ?? '',
        imageUrl = json['imageUrl'] ?? '',
        owner = json['owner'] ?? '';
}