Flutter postMessages not working on android emulator

I'm developing a webview app using flutter webview_flutter lib. When I send postMessages on IOS it works fine, but when testing on android emulator the terminal print an error message Uncaught Error: Method not found when executing the function postMessage.postMessage(JSON.stringify(obj), "*");.

Here is my webview code:

  WebView(
    javascriptMode: JavascriptMode.unrestricted,
    debuggingEnabled: true,
    initialUrl: '<url_to_my_template>',
    onPageStarted: (url) {},
    onPageFinished: (url) async {},
    javascriptChannels: <JavascriptChannel>{
      JavascriptChannel(
        name: 'postMessage',
        onMessageReceived: (message) async {
          print('postMessage: ${message.message}');
        },
      ),
    },
    onWebViewCreated: (WebViewController controller) {
      setState(() {
        _controller = controller;
      });
    },
  ),

And this is my js file:

function troca_dados_envia(obj){
    "use strict";
    //Flutter
    postMessage.postMessage(JSON.stringify(obj),  "*");
}

I tried the code above, and I was expecting to work on both Android and IOS devices, but it turns out that only IOS devices listen and send post messages.

We also went in circles with this, even though WebView on Android should support the full parameters of window.postMessage() it throws an error when defining the target origin.

Try just

postMessage.postMessage(JSON.stringify(obj));

Also for anybody else who gets stuck - you must ensure that the data being sent is a String, even though it seems other formats should be supported - it seems only Strings are sent reliably.