evalJavascript() won't work HTML DOM Methods

[rma6]

I'm trying to a input with text and submit a form.
When I press a button the following function is called

Future<void> _handleCNPJ() async {
    FlutterWebviewPlugin webView = new FlutterWebviewPlugin();

    await webView.launch(
      'http://www.receita.fazenda.gov.br/PessoaJuridica/CNPJ/cnpjreva/Cnpjreva_Solicitacao.asp',
      rect: new Rect.fromLTWH(0.0, 0.0, MediaQuery.of(context).size.width,
          MediaQuery.of(context).size.height / 2),
      userAgent: 'Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, 
          like Gecko) Chrome/62.0.3202.94 Mobile Safari/537.36',
      scrollBar: false,
      withZoom: false,
      supportMultipleWindows: false,
    );
    
    await webView.evalJavascript("document.getElementById('cnpj').value = 'testing'");
    await webView.close();
  }

But nothing happens on the WebView.
What am I doing wrong?

Thanks in advance.

[rma6]

I've found out that I wasn't waiting long enough to run the JS.

[erdemyerebasmaz]

@rma6
I don't know how you solved this but you can listen to state changes.
e.g.;

webView.onStateChanged.listen((state) async {
    if (state.type == WebViewState.finishLoad) {
        webView.evalJavascript("document.getElementById('cnpj').value = 'testing'");
    }
});

[rma6]

That's exactly what I did. Actualy I tried this approach first, but it wasn't working because I was initializing the listener in the wrong place. I had to put this code inside the initState()