Using postMessage in TWA

I have been looking at the relevant documentation:
https://developer.chrome.com/docs/android/post-message-twa
but I have not been able to implement this in my application.

[LauncherActivity.java](https://gist.github.com/DevNull-IR/14543d88e96fafa064a25af6c03ad869)

None of the Toasts are displayed except the first Toast, which is on line 57.

Thank you for your guidance in solving this problem.