how to send multi `windows.postmessage(data)` to a single source, with adding an ID to every message to be able to catch it

Question

I need to send more than one message using js to the same source flutter app is the source and it is ready to receive all messages based on their ID , which is mentioned in documentations to be sent like this :

measurePointTwo.postMessage(ptTwoData)

this causes the error :

Uncaught ReferenceError: measurePointTwo is not defined

That is because am using measurePointTwo and I cant not use this because this is how the listener in my flutter app is getting different messages, based on this ID or unique name.

What I have tried

window.measurePointTwo.postMessage(ptTwoData)

and this is getting me this error :

Uncaught TypeError: Cannot read properties of undefined (reading 'postMessage')

Extra info

• This window runs in my flutter app is a Web view

• Inside the WebView this function is used to handle postMessage :

                            javascriptChannels: {
                              JavascriptChannel(
                                  name: 'newCords',
                                  onMessageReceived:
                                      (JavascriptMessage message) {
                                    controller.getLagLngFromJSMessage(
                                        message.message);
                                  }),
                              JavascriptChannel(
                                  name: 'measurePointOne',
                                  onMessageReceived:
                                      (JavascriptMessage message) {
                                    controller.getPintOneFromJSMessage(
                                        message.message);
                                  }),
                              JavascriptChannel(
                                  name: 'measurePointTwo',
                                  onMessageReceived:
                                      (JavascriptMessage message) {
                                    controller.getPintTwoFromJSMessage(
                                        message.message);
                                  }),
                              JavascriptChannel(
                                  name: 'newPoint',
                                  onMessageReceived:
                                      (JavascriptMessage message) {
                                    controller.getClickedPoint(message.message);
                                  }),
                            },