How to properly dispose webview upon hot restart?

When using this plugin, I get an error upon hot-restart:`java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.`

```
[ +518 ms] E/MethodChannel#flutter_webview_plugin(18431): Failed to handle method call
[ ] E/MethodChannel#flutter_webview_plugin(18431): java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.view.ViewGroup.addViewInner(ViewGroup.java:4915)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.view.ViewGroup.addView(ViewGroup.java:4746)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.view.ViewGroup.addView(ViewGroup.java:4718)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at com.android.internal.policy.PhoneWindow.addContentView(PhoneWindow.java:468)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.app.Activity.addContentView(Activity.java:2702)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at com.flutter_webview_plugin.FlutterWebviewPlugin.openUrl(FlutterWebviewPlugin.java:143)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at com.flutter_webview_plugin.FlutterWebviewPlugin.onMethodCall(FlutterWebviewPlugin.java:53)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:233)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:692)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.os.MessageQueue.nativePollOnce(Native Method)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.os.MessageQueue.next(MessageQueue.java:325)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.os.Looper.loop(Looper.java:142)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at android.app.ActivityThread.main(ActivityThread.java:6541)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at java.lang.reflect.Method.invoke(Native Method)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
[ ] E/MethodChannel#flutter_webview_plugin(18431): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
[ +22 ms] E/flutter (18431): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: PlatformException(error, The specified child already has a parent. You must call removeView() on the child's parent first., null, java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
[ ] E/flutter (18431): at android.view.ViewGroup.addViewInner(ViewGroup.java:4915)
[ ] E/flutter (18431): at android.view.ViewGroup.addView(ViewGroup.java:4746)
[ ] E/flutter (18431): at android.view.ViewGroup.addView(ViewGroup.java:4718)
[ ] E/flutter (18431): at com.android.internal.policy.PhoneWindow.addContentView(PhoneWindow.java:468)
[ ] E/flutter (18431): at android.app.Activity.addContentView(Activity.java:2702)
[ ] E/flutter (18431): at com.flutter_webview_plugin.FlutterWebviewPlugin.openUrl(FlutterWebviewPlugin.java:143)
[ ] E/flutter (18431): at com.flutter_webview_plugin.FlutterWebviewPlugin.onMethodCall(FlutterWebviewPlugin.java:53)
[ ] E/flutter (18431): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:233)
[ ] E/flutter (18431): at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
[ ] E/flutter (18431): at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:692)
[ ] E/flutter (18431): at android.os.MessageQueue.nativePollOnce(Native Method)
[ ] E/flutter (18431): at android.os.MessageQueue.next(MessageQueue.java:325)
[ ] E/flutter (18431): at android.os.Looper.loop(Looper.java:142)
[ ] E/flutter (18431): at android.app.ActivityThread.main(ActivityThread.java:6541)
[ ] E/flutter (18431): at java.lang.reflect.Method.invoke(Native Method)
[ ] E/flutter (18431): at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
[ ] E/flutter (18431): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
[ ] E/flutter (18431): )
[ ] E/flutter (18431): #0 StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:582:7)
[ ] E/flutter (18431): #1 MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:159:18)
[ ] E/flutter (18431):
[ ] E/flutter (18431): #2 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:332:12)
[ ] E/flutter (18431): #3 FlutterWebviewPlugin.launch (package:flutter_webview_plugin/src/base.dart:225:20)
[ ] E/flutter (18431): #4 Api.launchWebview (package:encointer_wallet/service/substrateApi/api.dart:110:10)
[ ] E/flutter (18431): #5 Api.init (package:encointer_wallet/service/substrateApi/api.dart:52:5)
[ ] E/flutter (18431): #6 _WalletAppState._initStore (package:encointer_wallet/app.dart:93:14)
[ ] E/flutter (18431): #7 _rootRunUnary (dart:async/zone.dart:1198:47)
[ ] E/flutter (18431): #8 _CustomZone.runUnary (dart:async/zone.dart:1100:19)
[ ] E/flutter (18431): #9 _FutureListener.handleValue (dart:async/future_impl.dart:143:18)
[ ] E/flutter (18431): #10 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:696:45)
[ ] E/flutter (18431): #11 Future._propagateToListeners (dart:async/future_impl.dart:725:32)
[ ] E/flutter (18431): #12 Future._completeWithValue (dart:async/future_impl.dart:529:5)
[ ] E/flutter (18431): #13 _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:40:15)
[ ] E/flutter (18431): #14 _completeOnAsyncReturn (dart:async-patch/async_patch.dart:311:13)
[ ] E/flutter (18431): #15 AsyncAction.run (package:mobx/src/api/async/async_action.dart)
[ ] E/flutter (18431): #16 _rootRunUnary (dart:async/zone.dart:1198:47)
[ ] E/flutter (18431): #17 _CustomZone.runUnary (dart:async/zone.dart:1100:19)
[ ] E/flutter (18431): #18 _FutureListener.handleValue (dart:async/future_impl.dart:143:18)
[ ] E/flutter (18431): #19 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:696:45)
[ ] E/flutter (18431): #20 Future._propagateToListeners (dart:async/future_impl.dart:725:32)
[ ] E/flutter (18431): #21 Future._complete (dart:async/future_impl.dart:519:7)
[ ] E/flutter (18431): #22 new Future.microtask. (dart:async/future.dart:201:16)
[ ] E/flutter (18431): #23 _rootRun (dart:async/zone.dart:1182:47)
[ ] E/flutter (18431): #24 _CustomZone.run (dart:async/zone.dart:1093:19)
[ ] E/flutter (18431): #25 _CustomZone.runGuarded (dart:async/zone.dart:997:7)
[ ] E/flutter (18431): #26 _CustomZone.bindCallbackGuarded. (dart:async/zone.dart:1037:23)
[ ] E/flutter (18431): #27 _rootRun (dart:async/zone.dart:1190:13)
[ ] E/flutter (18431): #28 _CustomZone.run (dart:async/zone.dart:1093:19)
[ ] E/flutter (18431): #29 _CustomZone.runGuarded (dart:async/zone.dart:997:7)
[ ] E/flutter (18431): #30 _CustomZone.bindCallbackGuarded. (dart:async/zone.dart:1037:23)
[ ] E/flutter (18431): #31 _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
[ ] E/flutter (18431): #32 _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
[ ] E/flutter (18431):

```

I believe the problem is that hot-restart doesn't call `dispose()` where we close the webview.
https://github.com/encointer/encointer-wallet-flutter/blob/afc485de2ddf73b57d7b4956f418f3b1b44460f6/lib/app.dart#L121

Also `deactivate()` isn't called upon hot-restart

`reassemble()` is only called upon hot-reload (where we don't want to reload the webview), but not upon hot-restart

Bắt đầu với FlutterWebviewPlugin.openUrl tại vị trí trong Android stack trace và xem xét mã liên quan đến vòng đời được tham chiếu từ lib/app.dart. Theo dõi điều gì xảy ra khi ứng dụng hot-restart, bao gồm đường dẫn khởi chạy trong flutter_webview_plugin/src/base.dart. Được xem là hoàn tất khi hot-restart không còn cố gắng attach một WebView đã có parent và exception được báo cáo không còn xuất hiện.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.