function setOverlayHandler(handler){
    if(handler == null){
        document.getElementById('overlayAcknowledge').onclick = () => {
            toggleOverlay(false)
        }
    } else {
        document.getElementById('overlayAcknowledge').onclick = handler
    }
}
Set the onclick handler of the overlay acknowledge button.
If the handler is null, a default handler will be added.

@param {function} handler