The Media Session API standard evolves quite fast, so there's new actions that we might be able to support that the browser itself doesn't know yet, for instance the "stop" action.
By registering the callback to ourselves before calling into the browser, we're still able to handle it in the extension even if not supported by the browser.

The following now works:

navigator.mediaSession.setActionHandler("stop", () => {
    console.log("stop player");
});

Clicked Stop in context menu of media controller applet, got the callback executed.
Previously an exception would be thrown when calling setActionHandler