How to redirect a page using JS, based on the value of a <select> element

I want to replace a load of onchange="" inline JS actions attached to HTML elements. For example, I have a <select> that fires an inline onchange="" event that takes the selected option and redirects using window.document.location.href=''. I want something that passes CSP rules (no inline scripting) and uses .addEventListener. This is what I came up with. It works, but looks clunky. Is there a better way ?

window.addEventListener("DOMContentLoaded",EventListeners);

function EventListeners() {
  document.getElementById("MySelect").addEventListener("change", ChangeSomething);
}
function ChangeSomething() {
 var sel = document.getElementById("MySelect");
 var sel_opt = sel.options[sel.selectedIndex].id;
 window.document.location.href='page.php?id=' + sel_opt;
}

For some reason, if I try : document.getElementById("MySelect").addEventListener("change", ChangeSomething); as the first line, I get an error, saying getElementById(...) is null.