我收到此错误 getter 'length' was called on null Receiver: null : 尝试调用：length I/flutter

Posted 2023-02-23

【中文标题】我收到此错误 getter \'length\' was called on null Receiver: null : 尝试调用：length I/flutter

【英文标题】：I get this error The getter 'length' was called on null Receiver: null : Tried calling: length I/flutter我收到此错误 getter 'length' was called on null Receiver: null : 尝试调用：length I/flutter

【发布时间】：2020-12-15 03:45:52

【问题描述】：

import '../models/activity_model.dart';
import 'package:flutter/foundation.dart';

   class Trip 
   String id;
   String city;
  String name;
  List<Activity> ;
  DateTime date;
  Trip(
    @required this.name,
    @required this.city,
    @required this.activities,
    @required this.date,
  ) : id = UniqueKey().toString();


trip_activityList.dart

import 'package:flutter/material.dart';
import '../../../models/activity_model.dart';

class TripActivityList extends StatelessWidget 
  final List<Activity> activities;
  TripActivityList(this.activities);

  @override
  Widget build(BuildContext context) 
    return ListView.builder(
        itemCount: activities.length,
        itemBuilder: (context, index) 
          return Text(activities[index].name);
        );
  


trip_activities.dart

import 'package:flutter/material.dart';
import 'package:uvs_voyage/views/trip/widgets/trip_activity_list.dart';

import '../../../models/activity_model.dart';

class TripActivities extends StatelessWidget 
  final List<Activity> activities;
  TripActivities(this.activities);
  @override
  Widget build(BuildContext context) 
    return Container(
      child: DefaultTabController(
        length: 2,
        child: Column(
          children: <Widget>[
            Container(
              color: Colors.blue,
              child: TabBar(
                indicatorColor: Colors.blue[100],
                tabs: <Widget>[
                  Tab(
                    text: 'En cours',
                  ),
                  Tab(
                    text: 'Terminé',
                  ),
                ],
              ),
            ),
            Container(
              height: 600,
              child: TabBarView(
                physics: NeverScrollableScrollPhysics(),
                children: <Widget>[
                  TripActivityList(
                    activities: activities,
                  ),
                  TripActivityList(
                    activities: activities,
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  

这需要很长时间。）/I/flutter (15602): ══╡ 小部件库发现异常╞═════════════════════ ══════════════════════════════════════ I/flutter (15602)：在构建 TripActivityList(dirty) 时引发了以下 NoSuchMethodError： I/flutter (15602)：在 null 上调用了 setter 'length='。 I/颤振（15602）：接收器：空 I/flutter (15602)：尝试调用：长度=0 我/颤振（15602）： I/flutter (15602)：相关的导致错误的小部件是： 我/颤动（15602）：TripActivityList I/flutter (15602): file:///C:/Users/Mamadou%20Lamine%20Bouss/uvs_voyage/lib/views/trip/widgets/trip_activities.dart:35:19 我/颤振（15602）： I/flutter (15602)：抛出异常时，这是堆栈： I/flutter (15602): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5) I/flutter (15602): #1 TripActivityList.build (package:uvs_voyage/views/trip/widgets/trip_activity_list.dart:11:31) I/flutter (15602): #2 StatelessElement.build (package:flutter/src/widgets/framework.dart:4620:28) I/flutter (15602): #3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4546:15) I/flutter (15602): #4 Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5) I/flutter (15602): #5 StatelessElement.update (package:flutter/src/widgets/framework.dart:4627:5) I/flutter (15602): #6 Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15) I/flutter (15602): #7 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16) I/flutter (15602): #8 Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5) I/flutter (15602): #9 StatelessElement.update (package:flutter/src/widgets/framework.dart:4627:5) I/flutter (15602): #10 Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15) I/flutter (15602): #11 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5881:14) I/flutter (15602): #12 Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15) I/flutter (15602): #13 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5881:14) I/flutter (15602): #14 Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15) I/flutter (15602): #15 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16) I/flutter (15602): #16 Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5) I/flutter (15602): #17 StatelessElement.update (package:flutter/src/widgets/framework.dart:4627:5) I/flutter (15602): #18 Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15) I/flutter (15602): #19 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16) I/flutter (15602): #20 Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5) I/flutter (15602): #21 ProxyElement.update (package:flutter/src/widgets/framework.dart:4906:5) I/flutter (15602): #22 Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15) I/flutter (15602): #23 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)

【参考方案1】：

当 Widget 被实例化时，列表activities 可能为空。 当data == null时，你需要提防这种情况

 itemCount: activities!= null ? activities.length : 0,

【讨论】：

但我没有收到任何数据

您可能必须将其转换为可以刷新数据的有状态小部件，并在 initState 中将参数设置为 widget.activites