JavascriptChannel on iOS

I use JavascriptChannel to pass a string back to the app form webview.
It works perfectly for Android but can't make it work on iOS? Is there anything specific I need to do to make it work?

```
class WebViewJavascriptChannel {
static Set getChannel(Function(String) handler) {
return [
JavascriptChannel(
name: 'JavascriptChannel',
onMessageReceived: (JavascriptMessage javascriptMessage) {
handler(javascriptMessage.message);
print(javascriptMessage.message);
}),
].toSet();
}
}

Widget _getBankIDWebView(String url) {
return WebviewScaffold(
javascriptChannels:
WebViewJavascriptChannel.getChannel(_updateString),
url: url,
withZoom: false,
hidden: true,
appBar: _getAppBar(),
);
}

```

Start with WebviewScaffold and the javascriptChannels configuration shown in the issue, then reproduce the message flow on iOS and compare it with Android. Done means the issue documents a confirmed iOS-specific requirement or a reproducible failure with a scoped fix and verification steps.

Written by the indexing model from the issue text.