RangeError（索引）：無效值：有效值范圍為空：0 錯誤 Flutter

Question

你能告訴我我的代碼有什么問題以及如何避免在我的應用程序中顯示紅屏錯誤嗎？

════════ 小部件庫捕獲的異常═════════════════════════════␕══以下構建 ChatMessage(dirty) 時引發 RangeError：RangeError (index)：無效值：有效值范圍為空：0

我的代碼：

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

由於這條線，問題正在發生：

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

您已將變量聲明為：

String _name = '';

您正在嘗試使用索引訪問空字符串的第一個字符。 要解決此問題，請為該變量設置一些值：

String _name = 'Random';