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.

No file or test is named. Start by reproducing the flow through WebviewScaffold and the flutterWebviewPlugin.onStateChanged subscription: open the webview, go back, and reopen it. Done means a finishLoad event is received and prints abc on the second opening as well.

Written by the indexing model from the issue text.