Working on a document traversing plugin, I was required to build up a progress indicator in the UI. Basically traversing 

• all pages
• all componentSets
• all components

in this order.

I had communication working between code and ui, but from one day to another, this changed. Now I can get only message from UI to code, but not back. Code is sending, no matter what I am handing it for sending, “undefined” .. 

On UI side, this is awaiting the messages:

window.addEventListener("message", (event) => {
    console.log("Received " + event.data.postMessage)
    if (event === undefined) {
        console.error("undefined event sent, why?")
        return;
    }
    if (event.data === undefined) {
        console.error("undefined event.data sent, why?")
        return;
    }
    if (event.data.postMessage === undefined) {
        console.error("undefined event.data.postMessage sent, why?") 
        // ^ it breaks here, since postMessage is undefined
        return;
    }
    // .. actual code comes here, but not required, fails above already
});

On Code (backend) side I send it during a async execution .. 

async function GoAndGetThemTiger () {
  cancelExecution = false;
  let pageCurrentNumber = 0;
  for (const page of figma.root.children) {
    figma.ui.postMessage("TEST!!!!!"); // usually this worked
    await page.loadAsync();
  }
}
figma.showUI(__html__);
figma.ui.resize(500, 800);
// .. does not work
figma.ui.postMessage("Yes, let's kick of creation!");
// .. doesn't work neither
GoAndGetThemTiger();

What am I doing wrong here?

Any hints and remarks are welcome and happy to receive (unlike this message receiver )

Many thanks,

M