Flutter 错误：类型 Null 不是“字符串”类型的子类型

Question

I have faced this error while running my code, I'm trying to fetch data from api and display it into a simple widget (Center ),I'm posting the whole code model and the screen's code to fetch data and display it into a listview just to see that this method is working (knowing that I've tried it for other model and it worked ) This is the model's code:

import 'package:meta/meta.dart';
import 'dart:convert';

class PlanningCoach {
  final int? idPc;
  final double? prixPc;
  final DateTime? datePc;
  final String? horairePc;
  final int? nbpPc;
  final int? idcoach;
  final bool? recylcebin;
  PlanningCoach({
    required this.idPc,
    required this.prixPc,
    required this.datePc,
    required this.horairePc,
    required this.nbpPc,
    required this.idcoach,
    required this.recylcebin,
  });
  factory PlanningCoach.fromJson(Map<String, dynamic> json) => PlanningCoach(
        idPc: json["id_pc"],
        prixPc: json["prix_pc"],
        datePc: DateTime.parse(json["date_pc"]),
        horairePc: json["horaire_pc"],
        nbpPc: json["nbp_pc"],
        idcoach: json["idcoach"],
        recylcebin: json["recylcebin"],
      );
}

And this is code to fetch and display the data.

import 'dart:convert';
class DetailGroundScreen extends StatefulWidget {
  const DetailGroundScreen({Key? key}) : super(key: key);
  @override
    _DetailGroundScreenState createState() => _DetailGroundScreenState();
}
class _DetailGroundScreenState extends State<DetailGroundScreen> {
  late Future<List<PlanningCoach>> futurePCoach;
Future<List<PlanningCoach>> fetchPlanningCoach() async {
    final response = await http.get(Uri.parse(
        'http://smart.netrostercloud.com/smartcoach/api/plannningcoaches/displayPlanningCoach'));

    if (response.statusCode == 200) {
      List jsonResponse = json.decode(response.body);
      return jsonResponse
          .map((data) => new PlanningCoach.fromJson(data))
          .toList();
    } else {
      throw Exception('Failed to load coach');
    }
  }
@override
  void initState() {
    //TODO : implement initState
    super.initState();
    futurePCoach = fetchPlanningCoach();
    
  }
@override
  Widget build(BuildContext context) {
    final value = ModalRoute.of(context)!.settings.arguments;
// ignore: unnecessary_new
    return Center(
      child: FutureBuilder<List<PlanningCoach>>(
        future: futurePCoach,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            List<PlanningCoach>? data = snapshot.data;
            return ListView.builder(
                shrinkWrap: true,
                itemCount: data?.length,
                itemBuilder: (BuildContext context, int index) {
                  return Container(
                    height: 75,
                    color: Colors.white,
                    child: Center(
                      child: Text('${data![index].idPc}'),
                    ),
                  );
                });
          } else if (snapshot.hasError) {
            return Text("${snapshot.error}");
          }
          // By default show a loading spinner.
          return CircularProgressIndicator();
        },
      ),
    );


}
}

This is the error:

Answer 1

From

  horairePc: json["horaire_pc"],

To

  horairePc: json["horaire_pc"].toString(),

Answer 2

You have couple of issues in your code. I will try pointing as many as I can:

1. Null-saftey usage:

    List<PlanningCoach>? data = snapshot.data; return ListView.builder( shrinkWrap: true, itemCount: data?.length, itemBuilder: (BuildContext context, int index) { return Container( height: 75, color: Colors.white, child: Center( child: Text('${data.[index],idPc}'), ); ); });

data is decalred as a nullable field meaning, data can be null. so while access data it will be a good idea to access with ? . idPc also can be null. However a Text() always expects a string field. Change the code to below:

Text('${data?.elementAt(index).idPc ?? ''}') // if null use empty string

2. Make the parent most page a Scaffold : Currently your screen shows all black because there is no proper root widget that can be used at the page level eg Scaffold. Make you FutureBuilder a child to Scaffold

3. DateTime.parse() in PlanningCoach: THe code datePc: DateTime.parse(json["date_pc"]), will throw an excption if json["date_pc"] is null. so a better idea would be to check for null and then access.

    if(json["date_pc"]) datePc: DateTime.parse(json["date_pc"])

Probably this is causing the null exception.

Answer 3

perhaps the fact is that the name of the key is written in json not as "id_pc", but in a different way