Calling dispose() and close() within onUrlChanged does not pop the WebviewScaffold

When trying to pop the WebviewScaffold by calling `FlutterWebViewPlugin.dispose()` and `FlutterWebViewPlugin.close()` after having been redirected, instead of popping the Route, the `_WebviewPlaceholder` only switches to the `child` Widget (`initalChild` or `CircularProgressIndicator`), rather than returning to the previous Route.