Flutter-将getter'length'调用为null

Question

我正在尝试开发一个Flutter应用程序。这种动荡正在为纸牌游戏创建团队。团队创建后，积分可以计算在内，因此您不必考虑每个人有多少积分。

但是我有一个例外，我知道例外在哪里及其含义，但是我不知道如何解决该问题。我希望你们中的一些人能帮助我。

这是引发错误的代码：

import 'package:flutter/material.dart';


class Punktezaehler extends StatefulWidget{

  final List<String> spieler_namen;
  Punktezaehler(this.spieler_namen);

  @override
  State<StatefulWidget> createState() => new _Punktezaehler(this.spieler_namen);
}


class _Punktezaehler extends State<Punktezaehler>{

  final List<String> spieler_namen;
  _Punktezaehler(this.spieler_namen);

  List<int> punkteanzahl_teamEins;
  List<int> punkteanzahl_teamZwei;

  @override
  Widget build(BuildContext context) {


    var spieler1 = spieler_namen[0].substring(0,3);
    var spieler2 = spieler_namen[1].substring(0,3);
    var spieler3 = spieler_namen[2].substring(0,3);
    var spieler4 = spieler_namen[3].substring(0,3);

    return new Scaffold(
      appBar: new AppBar(
        automaticallyImplyLeading: false,
        title: new Text("$spieler1 & $spieler2 vs" +" $spieler3 & $spieler4"),
        actions: <Widget>[

        ],
      ),
      body: Container(
        child: new Row(
          children: <Widget>[
            new Column(
              children: <Widget>[
                new IconButton(
                    icon: Icon(Icons.exposure_plus_2),
                    onPressed: () => punkte_hinzuzaehlen(1, 2)
                )
              ],
            ),
            new Column(
              children: <Widget>[
                //new FlatButton(onPressed: () => print(punkteanzahl_teamEins.length), child: new Text("Punkte")),
                ListView.builder(
                  itemCount: punkteanzahl_teamEins.length, //--> Error is thrown here
                  itemBuilder: (context, index){
                    return Text(punkteanzahl_teamEins[index].toString());
                  }
                ),
                new Row()
              ],
            ),
            new Column(
              children: <Widget>[
                new IconButton(
                    icon: Icon(Icons.exposure_plus_2),
                    onPressed: () => punkte_hinzuzaehlen(2, 2)
                )],
            )
          ],
        )
      ),
    );
  }

  void punkte_hinzuzaehlen(int team, int nummer){
    if (team == 1){
      //Team 1 bekommt die Punkte
      print("Team 1 gets points");
    }
    else if(team == 2){
      //Team 2 bekommt die Punkte
      print("Team 2 gets points");
    }
  }
}

这是错误消息：

??? EXCEPTION CAUGHT BY GESTURE ????????????????????????????????????????????????????????????????????
I/flutter (26028): The following NoSuchMethodError was thrown while handling a gesture:
I/flutter (26028): The getter 'length' was called on null.
I/flutter (26028): Receiver: null
I/flutter (26028): Tried calling: length

修复后，我得到另一个错误：

??? EXCEPTION CAUGHT BY RENDERING LIBRARY ??????????????????????????????????????????????????????????
I/flutter (26028): The following assertion was thrown during performResize():
I/flutter (26028): Vertical viewport was given unbounded width.
I/flutter (26028): Viewports expand in the cross axis to fill their container and constrain their children to match
I/flutter (26028): their extent in the cross axis. In this case, a vertical viewport was given an unlimited amount of
I/flutter (26028): horizontal space in which to expand.

Answer 1

仅声明punkteanzahl_teamEins。但未初始化。因此，它引发空错误。

您应该将值分配punkteanzahl_teamEins为

List<int> punkteanzahl_teamEins = [1,4,5,7];

或根据需要传递来自父级的数据。