RangeError（索引）：无效值：有效值范围为空：0错误Flutter

Question

您能否告诉我我的代码有什么问题以及如何避免在我的应用中显示红屏错误？

════════ 小部件库捕获的异常════════════════════════════════ ═ 构建 ChatMessage(dirty) 时引发了以下 RangeError： RangeError（索引）：无效值：有效值范围为空：0

我的代码：

import 'package:crypto_app/models/users.dart';
import 'package:crypto_app/pages/login.dart';
import 'package:crypto_app/services/firestoreservice.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:meta/meta.dart';

void main() {
  runApp(
    FriendlyChatApp(),
  );
}

final ThemeData kIOSTheme = ThemeData(
  primarySwatch: Colors.blue,
  primaryColor: Colors.grey[100],
  primaryColorBrightness: Brightness.light,
);

final ThemeData kDefaultTheme = ThemeData(
  primarySwatch: Colors.orange,
  accentColor: Colors.orangeAccent,
);

String _name = '';

class FriendlyChatApp extends StatelessWidget {
  const FriendlyChatApp({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: ChatScreen(),
    );
  }
}

class ChatMessage extends StatelessWidget {
  ChatMessage({this.text, this.animationController});
  final String text;
  final AnimationController animationController;

  @override
  Widget build(BuildContext context) {
    return SizeTransition(
      sizeFactor:
          CurvedAnimation(parent: animationController, curve: Curves.easeOut),
      axisAlignment: 0.0,
      child: Container(
        margin: EdgeInsets.symmetric(vertical: 10.0),
        child: Row(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Container(
              margin: const EdgeInsets.only(right: 16.0),
              child: CircleAvatar(child: Text(_name[0])),
            ),
            Expanded(
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: [
                  Text(_name, style: Theme.of(context).textTheme.headline4),
                  Container(
                    margin: EdgeInsets.only(top: 5.0),
                    child: Text(text),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class ChatScreen extends StatefulWidget {
  @override
  _ChatScreenState createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> with TickerProviderStateMixin {
  final List<ChatMessage> _messages = [];
  final _textController = TextEditingController();
  final FocusNode _focusNode = FocusNode();
  bool _isComposing = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        decoration: Theme.of(context).platform == TargetPlatform.iOS //new
            ? BoxDecoration(
                border: Border(
                  top: BorderSide(color: Colors.grey[200]),
                ),
              )
            : null,
        child: Column(
          children: [
            Flexible(
              child: ListView.builder(
                padding: EdgeInsets.all(8.0),
                reverse: true,
                itemBuilder: (_, int index) => _messages[index],
                itemCount: _messages.length,
              ),
            ),
            Divider(height: 1.0),
            Container(
              decoration: BoxDecoration(color: Theme.of(context).cardColor),
              child: _buildTextComposer(),
            ),
          ],
        ),
      ),
    );
  }

  Widget _buildTextComposer() {
    return IconTheme(
      data: IconThemeData(color: Theme.of(context).accentColor),
      child: Container(
        margin: EdgeInsets.symmetric(horizontal: 8.0),
        child: Row(
          children: [
            Flexible(
              child: TextField(
                controller: _textController,
                onChanged: (String text) {
                  setState(() {
                    _isComposing = text.isNotEmpty;
                  });
                },
                onSubmitted: _isComposing ? _handleSubmitted : null,
                decoration: InputDecoration.collapsed(
                    hintText: 'Mesajınızı Buraya Yazınız:'),
                focusNode: _focusNode,
              ),
            ),
            Container(
                margin: EdgeInsets.symmetric(horizontal: 4.0),
                child: Theme.of(context).platform == TargetPlatform.iOS
                    ? CupertinoButton(
                        onPressed: _isComposing
                            ? () => _handleSubmitted(_textController.text)
                            : null,
                        child: Text('Gönder'),
                      )
                    : IconButton(
                        icon: const Icon(Icons.send),
                        onPressed: _isComposing
                            ? () => _handleSubmitted(_textController.text)
                            : null,
                      ))
          ],
        ),
      ),
    );
  }

  void _handleSubmitted(String text) {
    _textController.clear();
    setState(() {
      _isComposing = false;
    });
    var message = ChatMessage(
      text: text,
      animationController: AnimationController(
        duration: const Duration(milliseconds: 700),
        vsync: this,
      ),
    );
    setState(() {
      _messages.insert(0, message);
    });
    _focusNode.requestFocus();
    message.animationController.forward();
  }

  @override
  void dispose() {
    for (var message in _messages) {
      message.animationController.dispose();
    }
    super.dispose();
  }
}

Answer 1

问题是因为这条线而发生的：

CircleAvatar(child: Text(_name[0])),

您已将变量声明为：

String _name = '';

您正在尝试使用索引访问空字符串的第一个字符。要解决此问题，请为该变量设置一些值：

String _name = 'Random';