i wrote a JS code for my Website, which should show a modal and close it, if i click the "close" span. The first time, it worked perfectly. Here is the code:

var modal = document.getElementById("notifications");
var img = document.getElementById("shownotificationbar");
var span = document.getElementsByClassName("close")[0];

img.onclick = function() {
  modal.style.display = "block";
}

span.onclick = function() {
  modal.style.display = "none";
} 

But the second time, the close button just dont work and i get

"Uncaught TypeError: Cannot set properties of null (setting 'onclick')"

Here is the second code piece:

var menu = document.getElementById("startmenu");
var img = document.getElementById("showstartmenu");
var span = document.getElementById("closemenu");


img.onclick = function() {
  menu.style.display = "block"; 
}

span.onclick = function() {
  menu.style.display = "none";
}

I think i can't use two spans, but what else should i do? Thank you very much!