i was taking a Flutter course (the course was recorded before flutter 2) when i ran into this error:

I/flutter ( 3538): type 'Null' is not a subtype of type 'String'
E/flutter ( 3538): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: type 'Null' is not a subtype of type 'String'
E/flutter ( 3538): #0      Products.fetchAndSetProducts
package:shop_app/providers/products_provider.dart:80
E/flutter ( 3538): <asynchronous suspension>

I tried to check if there was something wrong on the line highlighted and for me it was ok.

Here is the code:

Future<void> fetchAndSetProducts() async {
    var url = Uri.parse(
        'https://flutter-39ecc-default-rtdb.firebaseio.com/products.json');
    try {
      final response = await http.get(url);
      final extractedData = json.decode(response.body) as Map<String, dynamic>;
      final List<Product> loadedProducts = [];
      extractedData.forEach((key, value) {
        loadedProducts.add(Product(
            id: key,
            title: value['title'],
            description: value['description'],
            price: value['price'],
            isFavorite: value['isFavorite'],
            imageUrl: value['imageUrl']));
      });
      _items = loadedProducts;
      notifyListeners();
      // print(json.decode(response.body));
    } catch (error) {
      print(error);
      throw error;  // line 80
    }
  }

This is how i´m calling this function

  var _isInit = true;
  var _isLoading = false;
  @override
  void didChangeDependencies() {
    if (_isInit) {
      setState(() {
        _isLoading = true;
      });
      Provider.of<Products>(context).fetchAndSetProducts().then((_) {
        setState(() {
          _isLoading = false;
        });
      });
    }
    _isInit = false;
    super.didChangeDependencies();
  }

CodePudding user response：

Please check your Product class. Its attributes should be able to be Null.

So it's may be String? not String

  Product(
        id: key,
        title: value['title'],
        description: value['description'],
        price: value['price'],
        isFavorite: value['isFavorite'],
        imageUrl: value['imageUrl']));
  }

I checked the response from https://flutter-39ecc-default-rtdb.firebaseio.com/products.json. There are some missing keys that need to use when construct the Product object.

CodePudding user response：

this is happenned because one of the value you're trying to assign is null while the variable is not nullable

        loadedProducts.add(Product(
            id: key,
            title: value['title'],
            description: value['description'],
            price: value['price'],
            isFavorite: value['isFavorite'],
            imageUrl: value['imageUrl']));
      });

look at this code, if you want to make a model class of product, it would be great if you make every variable nullable.

class ProductModel {
  int? id;
  String? title;
  String? description;
  double? price;
  bool? isFavorite;
  String? imageUrl;
}

And it would be great if you really careful with the assignment from the backend. Because some backend aren't consistent.

factory ProductModel ProductModel.fromJson(Map<String, dynamic> json) => ProductModel({
  id: json['id'] != null ? (json['id'] as num?)?.toInt() : null, 
//it would be great if you also check if the json['id'] is instance of String or not, because sometimes you can get id with type String from backend
// why use num? instead of int?, because sometimes we can also get double from backend right? and int and double indicator isn't consistent
// that's why num? then cast to the type you want is the best practice
  title: json['title'] as String?,
  
  
  //etc..
});