iOS app crashes when speaking different languages quickly and alternately

[nero-angela]

🐛 Bug Report

iOS app crashes when speaking different languages quickly and alternately

When running with the flutter project, the error log was not output, so it was run with xcode.

https://user-images.githubusercontent.com/26322627/105464974-b20e4080-5cd5-11eb-9725-108d3dda42f4.mp4

Reproduction steps

1. add below code and run
2. quickly press the two buttons alternately

   
   import 'package:flutter/material.dart';
   import 'package:flutter_tts/flutter_tts.dart';

void main() { runApp(MyApp()); }

class MyApp extends StatefulWidget { MyApp({Key key}) : super(key: key);

@override _MyAppState createState() => _MyAppState(); }

class _MyAppState extends State { final FlutterTts tts = FlutterTts(); String currentLanguage;

@override void initState() { super.initState(); }

void speak(String text, String lang) async { if (currentLanguage != lang) { await tts.setLanguage(lang); currentLanguage = lang; } tts.speak(lang); }

@override Widget build(BuildContext context) { return MaterialApp( home: Scaffold( body: Column( mainAxisAlignment: MainAxisAlignment.center, crossAxisAlignment: CrossAxisAlignment.stretch, children: [ RaisedButton( onPressed: () => speak('hello world', 'en-US'), child: Text('speak english'), ), RaisedButton( onPressed: () => speak('안녕하세요', 'ko-KR'), child: Text('speak korean'), ), ], ), ), ); } }

### Configuration

**Version:** 2.1.0

**Platform:** 
- [x] :iphone: iOS
- [ ] :robot: Android

[nero-angela]

In the latest version, the issue does not seem to occur, so I close it.