Flutter - The method 'map' was called on null

Question

I'm receiving the following error while trying to add elements from my api JSON using Object Model to DropdownMenuItem

here is the error :

The method 'map' was called on null.
Receiver: null
Tried calling: map<DropdownMenuItem<Provinces>>(Closure: (Provinces) => DropdownMenuItem<Provinces>)

here is my dart code :

import 'dart:async';

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:sj/utils/constants.dart';
import 'package:http/http.dart' as http;
import 'package:sj/models/master/provinces.dart';

class IdCardFormPage extends StatefulWidget {
  @override
  _IdCardFormPageState createState() => new _IdCardFormPageState();
}

class _IdCardFormPageState extends State<IdCardFormPage> {

  List<Provinces> listProvinces;
  Provinces _selectedProvince;

  Future<List<Provinces>> getProvinceList() async {
    //final response = await http.get("${APIConstants.API_BASE_URL}api/masters/provincesList.php");
    final response = await http.get("url");

    listProvinces = parseProvinces(response.body);

    return parseProvinces(response.body);
  }

  List<Provinces> parseProvinces(String responseBody) {
    final parsed = json.decode(responseBody).cast<Map<String, dynamic>>();

    return parsed.map<Provinces>((json) => Provinces.fromJson(json)).toList();
  }


  @override
  void initState() {

    getProvinceList();

    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: new Text("MASUK"),),
      body: _provinceContainer(),
    );
  }

  Widget _provinceContainer(){
    return new Container(
        child: new InputDecorator(
          decoration: InputDecoration(
              suffixIcon: new Icon(
                Icons.account_balance,
                color: Colors.pink,
              ),
              labelText: LoanEntryField.PD_BANKNAME, labelStyle: TextStyle(fontSize: 16.0)
          ),
          isEmpty: _selectedProvince == null,
          child: new DropdownButtonHideUnderline(
            child: new DropdownButton<Provinces>(
              value: _selectedProvince,
              isDense: true,
              onChanged: (Provinces newValue) {
                setState(() {
                  _selectedProvince = newValue;
                });
              },
              items: listProvinces.map((Provinces value) {
                return new DropdownMenuItem<Provinces>(
                  value: value,
                  child: new Text(value.name, style: new TextStyle(fontSize: 16.0),),
                );
              }).toList(),
            ),
          ),
        ),
        margin: EdgeInsets.only(bottom: 10.0));
  }

}

class Provinces{
  String id;
  String name;

  Provinces({this.id, this.name});

  factory Provinces.fromJson(Map<String, dynamic> json) {
    return Provinces(
      id: json['id'] as String,
      name: json['name'] as String,
    );
  }

}

How to solve this issue?

Answer 1

Pass an empty container while listProvices is null:

items: listProvices?.map((Provinces value) {
            return new DropdownMenuItem<Provinces>(
              value: value,
              child: new Text(value.name, style: new TextStyle(fontSize: 16.0),),
            );
          })?.toList() ?? [],

Answer 2

Got the same issue while trying to pass a without assign value list to the items property of the DorpdownMenu widget.

The error message The method 'map' was called on null. is indicating you that you've called the index [ ] operator on null.

So before using the List, assign a some value to it List or Initialize the List.

to I did Initialize the List and it got fixed, following code

List<Provinces> listProvinces = [];

It will still work while containerer doesn't compile