Flutter Widget Tree嵌入WebView会出现白屏问题

### 运行环境

**Flutter Version** 2.0.6
**Flutter Boost Version** 最新的master分支
**Target Platform:** Android
**Target OS version/browser:** Android 11
**Devices:** Pixel 3

运行附件的example=>点击首页的“打开Flutter WebView”=>按返回键=>重新点击“打开Flutter WebView”

现象：
第二次打开WebView白屏

我的App在嵌入WebView的页面打开下一个页面返回后，嵌入WebView的页面才会出现白屏，退出重新打开不会白屏，跟附件的Demo运行效果不一样。

【备注】纯Flutter开发不会出现此问题

[flutter_boost-webview.zip](https://github.com/alibaba/flutter_boost/files/6478079/flutter_boost-webview.zip)

Start with the attached flutter_boost-webview.zip example and reproduce the Android 11 flow on a Pixel 3: open Flutter WebView, press Back, then open it again. Compare this with the reported app flow and verify that reopening the WebView no longer produces a white screen.

Written by the indexing model from the issue text.