evaluate javascript takes too long in flutter webview?

Question

I am trying to build a flutter app and I am trying to show up website in a webview I am using "flutter_webview_plugin: ^0.3.11" now I am trying to evaluate some javascript in order to delete some items before displaying the web page here is the code of the webview:

    @override
Widget build(BuildContext context) {
return WebviewScaffold(
  url: 'https://www.amazon.com',
  appBar: AppBar(
    title: Text('Amazon store'),
  ),
  initialChild: WebviewAnimation(),
  withJavascript: true,
);

and here is the code of evaluating I am listening to onStateChanged like below:

void initState() {
super.initState();
_onUrlChanged = flutterWebViewPlugin.onUrlChanged.listen((event) {
  this.url = event;
});
_onStateChanged =
    flutterWebViewPlugin.onStateChanged.listen((WebViewStateChanged state) {
  if (mounted) {
    setState(() {
      flutterWebViewPlugin.hide();
      var kiki = flutterWebViewPlugin.evalJavascript(
          "document.getElementsByClassName(\"a-button-stack\")[0].style.display='none';");
      _history.add('onStateChanged: ${state.type} ${state.url}');
      kiki.then((value) {
        setState(() {
          flutterWebViewPlugin.show();
        });
      });
    });
  }
});

by doing this I am able to evaluate js, the problem is it takes too long to evaluate, therefore I want to show up the page once it has been evaluated...any ideas ...thanks in advance