Outlook Add - Update TaskPane Elements on OnMessageRecipientsChanged event

Question

I am trying to update TaskPane HTML elements upon event OnMessageRecipientsChanged gets fired. The event is received successfully however, I am unable to access HTML elements using:

document.getElementById("item-subject").innerHTML = "Testing..."

Also, this element is available in Office.OnReady() and I am able to update it there but I instead want to do this onMessageRecipientsChanged event.

This works:

Office.onReady(function () {
  $(document).ready(function () {
    document.getElementById("item-subject").innerHTML = "Testing...";
  });
});

This doesn't work:

function onMessageRecipientsChangedHandler(event) {
  document.getElementById("item-subject").innerHTML = "Testing...";
}

How can I get this to work?