Flutter-方法“ map”在null上被调用 [英] Flutter - The method 'map' was called on null

问题描述

我在尝试使用对象模型将API JSON中的元素添加到 DropdownMenuItem

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

这是错误：

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

这是我的飞镖代码：

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,
    );
  }

}

如何解决此问题？ / p>

How to solve this issue?

推荐答案

在 listProvices 为时传递一个空容器null ：

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

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