Flutter - NoSuchMethodError：方法'visitChildren'在null上被调用

Question

我是新来的颤振和从 youtube 学习的人。我试图构建一个加密货币应用程序，它将显示从https://api.coinmarketcap.com/v1/ticker/?limit=50 获取数据的加密货币的价格和名称。我写的代码没有任何编译错误。

VS CODE 没有给我任何错误。但是当我在 web 和 Android 上运行应用程序时，它会抛出一个错误，上面写着： NoSuchMethodError：方法 'visitChildren' 在 null 上被调用。 收件人：空 尝试调用：visitChildren(Clousre:(InlineSpan)=>bool)

我有两个文件 1. main.dart

import 'package:flutter/material.dart';
import 'HomePage.dart';
import 'package:http/http.dart' as http;

void main() async {
  List currencies = await getCurrencies();
  print(currencies);
  runApp(new MyApp(currencies));
  }

class MyApp extends StatelessWidget {
  final List currencies;
  MyApp(this.currencies);
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: new ThemeData(primarySwatch: Colors.yellow),
        home: new HomePage(currencies),
    );
  }
}
Future<List> getCurrencies() async{
    String apiUrl = "https://api.coinmarketcap.com/v1/ticker/?limit=50";
    http.Response response = await http.get(apiUrl);
    return jsonDecode(response.body);
  }

1. HomePage.dart

import 'package:flutter/material.dart';

class HomePage extends StatefulWidget {
  final List currencies;
  HomePage(this.currencies);
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {

final List<MaterialColor> colors =[Colors.green, Colors.blue, Colors.red];
List currencies = [];
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: new AppBar(
        title: new Text("Crypto Currency")
      ),
      body: cryptoWidget(),
    );
  }

  Widget cryptoWidget(){
    return Container(
      child: new Column(
      children: <Widget>[
        new Flexible(
          child: new ListView.builder(
            itemCount: widget.currencies.length,
            itemBuilder: (BuildContext context, int index){
              final Map currency = widget.currencies[index];
              final MaterialColor color = colors[index % colors.length]; 
              return getListItemUI(currency, color);
            },
          ),
        ),],
      ),
    );
  }
  ListTile getListItemUI(Map currency, MaterialColor color){
    return new ListTile(
      leading: CircleAvatar(
        backgroundColor: color,
        child: Text(currency['name'][0]),
      ),
      title: Text(currency['name'],
      style: TextStyle(
        fontWeight: FontWeight.bold)),
      subtitle: getSubtitleText(
        currency['price_usd'], currency['percent_change_1h']),
      isThreeLine: true,   
    );
  }
    Widget getSubtitleText(String priceUSD, String percentChange){
      TextSpan priceTextWidget = TextSpan(
        text: "\$$priceUSD\n", style: TextStyle(color: Colors.black)
      );

      String percentageChange = "1 hour $percentChange%";
      TextSpan percentageChangeText;

      if(double.parse(percentChange)>0){
        //TODO 1: Select That text
        priceTextWidget = new TextSpan(
          text: percentageChange, 
          //TODO 2: Change its color to green
          style: TextStyle(color: Colors.green));
      }

      else{
//TODO 1: Select That text
        priceTextWidget = new TextSpan(
          text: percentageChange, 
          //TODO 2: Change its color to green
          style: TextStyle(color: Colors.red));
      }

      return RichText(
        text: TextSpan(
          children: [priceTextWidget, percentageChangeText]));
    }
  }

Answer 1

您将 percentageChangeText 放入 TextSpan 的子项中，但 percentageChangeText 为空。

      TextSpan percentageChangeText;

      // ...

      return RichText(
        text: TextSpan(
          children: [priceTextWidget, percentageChangeText]));