It seems that document.getElementById('overlayBtn'); is returning null because it executes before the DOM fully loads.
If you put this line of code under
window.onload=function(){
-- put your code here
}
then it will run without issue.
Example:
window.onload=function(){
var mb = document.getElementById("b");
mb.addEventListener("click", handler);
mb.addEventListener("click", handler2);
}
function handler() {
$("p").html("<br>" + $("p").text() + "<br>You clicked me-1!<br>");
}
function handler2() {
$("p").html("<br>" + $("p").text() + "<br>You clicked me-2!<br>");
}