Fix for "Future already completed" error in web

[abhimanyupandian]

New Pull Request Checklist

• [x] I have read the Documentation
• [x] I have searched for a similar pull request in the project and found none
• [x] I have updated this branch with the latest develop to avoid conflicts (via merge from master or rebase)
• [x] I have added the required tests to prove the fix/feature I am adding
• [x] I have updated the documentation (if necessary)
• [x] I have run the tests and they pass

This merge request fixes / refers to the following issues: https://github.com/flutterchina/dio/issues/1497

Pull Request Description

The completer is called without checking if it is already complete, so a "Bad State" completer already completed error is thrown.

[abhimanyupandian]

@wendux could you please approve this PR so that workflows can run?

[bjzhangyongbin]

1、环境：Web 浏览器：Chrom 102.0.5005.63（正式版本） （64 位） Flutter SDK:：flutter_windows_3.0.2-stable.zip Android Studio：2021

2、现象： 1）BaseOptions有connectTimeout参数，执行Get时，从Api后台返回数据成功，然后报错，错误信息如下

errors.dart:251 Uncaught (in promise) Error: Bad state: Future already completed at Object.throw_ [as throw] (errors.dart:251:49) at _AsyncCompleter.new.completeError (future_impl.dart:15:79) at browser_adapter.dart:70:22 at _RootZone.runUnary (zone.dart:1685:54) at _FutureListener.then.handleValue (future_impl.dart:147:18) at handleValueCallback (future_impl.dart:766:44) at _Future._propagateToListeners (future_impl.dart:795:13) at [_complete] (future_impl.dart:557:7) at future.dart:420:15 at internalCallback (isolate_helper.dart:48:19)

2）BaseOptions没有connectTimeout参数时，返回数据成功，没有报错。

3、代码： ` import 'package:dio/dio.dart';

class _DioTestState extends State<DioTest> { @override Widget build(BuildContext context) { return Wrap( children: <Widget>[ ElevatedButton( onPressed: () { getData() ;}, child: const Text('test get'), ) ], ); }

void getData() { BaseOptions options = BaseOptions( baseUrl: 'http://127.0.0.1:9001', connectTimeout: 5000, receiveTimeout: 3000, ); Dio dio = Dio(options);

dio.get('/openApi/resTest/no_need_token').then((res) {
  print(res.toString());
}).catchError((e) {
  print(e.toString());
});

} } `

[manabu0926]

@wendux When will this PR be merged? I've been getting errors all the time and I'm in great trouble. I would be very happy if you could support me!

[HugoHeneault]

Unsure that this fix is enough, I've tried it and keep getting (less) errors

[gslender]

Fixed it for me. Would be good to see this PR pushed as these web bugs are annoying (even if they don't stop anything)