onStateChanged listener listening only once

flutter_webview_plugin: ^0.3.8

I open the webview when clicking on a button using:
```
Navigator.push(
context,
MaterialPageRoute(
builder: (BuildContext context) => WebviewScaffold(
url: 'https://google.com',
appBar: AppBar(
title: Text('WebView'),
),
),))
```
In my build method I have
```
flutterWebviewPlugin.onStateChanged.listen(
(WebViewStateChanged viewState) async {
if (viewState.type == WebViewState.finishLoad) {
print('abc');
}
});
```

The onStateChanged works perfectly if I just go from one page to the next but If I hit the back Arrow in the AppBar and again click on the button to open webview, the breakpoint hits 'flutterWebviewPlugin.onStateChanged.listen' line in the build method but it doesn't go inside the onStateChanged.listen function and doesn't print abc.

未指定文件或测试。首先通过 WebviewScaffold 和 flutterWebviewPlugin.onStateChanged 订阅复现该流程：打开 webview，返回，然后重新打开。完成的标准是第二次打开时同样能收到 finishLoad 事件并打印 abc。

由索引模型根据 Issue 内容生成。