i want to call custom flutter function from javascription fuction.

Here is my code.
MainActivity.kt
override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
GeneratedPluginRegistrant.registerWith(flutterEngine);
}

WebviewManager.java
webView.addJavascriptInterface(new WebAppInterface(), "Android");
//////////////
public class WebAppInterface {
@JavascriptInterface
public void getPostMessage(String value){
Map postMessageMap = new HashMap<>();
postMessageMap.put("postMessage", value);
FlutterWebviewPlugin.channel.invokeMethod("onPostMessage", postMessageMap);
}
}

dart file :
flutterWebViewPlugin.onStateChanged.listen((state) async {

if(state.type == WebViewState.finishLoad) {

String script = 'window.addEventListener("message", sendWhatsApp,false);' +
'function sendWhatsApp(msg) {Android.postMessage("data");}';

flutterWebViewPlugin.evalJavascript(script);

}