This code works on jsfiddle.net, but not on SquareSpace. What am I doing wrong?

Site URL: https://www.firesideediting.com.au/test-page

I have the following in Code Injection:

<script>
  
var popupArray = [];
//Build the array of all popups on the page
for (let i = 1; i < 20; i++) {
  let idString = "myPopup";
  idString += i;
  if (document.getElementById(idString)) {
    popupArray.push(document.getElementById(idString));
  }
}
var popupLength = popupArray.length;

function myFunction(whichTooltip) {
  var clickedPopup = document.getElementById(whichTooltip);
  //if clicking on one that has a popup, just close it
  if (clickedPopup.classList.contains("show")) {
    clickedPopup.classList.remove("show");
  } else {
    //if clicking on any other, stop showing current popups, and instead show the one clicked
    closeAllPopups();
    clickedPopup.classList.toggle("show");
  }
}

function closeAllPopups() {
  for (let i = 0; i < popupLength; i++) {
    popupArray[i].classList.remove("show");
  }
}

</script>

This in Custom CSS:

.popup {
  position: relative;
  display: inline-block;
  cursor: pointer;
  color: blue;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

/* The actual popup */
.popup .popuptext {
  visibility: hidden;
  width: 160px;
  background-color: #555;
  color: #fff;
  text-align: center;
  border-radius: 6px;
  padding: 8px 0;
  position: absolute;
  z-index: 1;
  bottom: 125%;
  left: 50%;
  margin-left: -80px;
}

/* Popup arrow */
.popup .popuptext::after {
  content: "";
  position: absolute;
  top: 100%;
  left: 50%;
  margin-left: -5px;
  border-width: 5px;
  border-style: solid;
  border-color: #555 transparent transparent transparent;
}

/* Toggle this class - hide and show the popup */
.popup .show {
  visibility: visible;
  -webkit-animation: fadeIn 1s;
  animation: fadeIn 1s;
}

/* Add animation (fade in the popup) */
@-webkit-keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

And this in HTML:

<p><span class="popup" onclick="myFunction('myPopup1')">Click me to toggle the first popup!<span class="popuptext" id="myPopup1">Popup 1</span></span></p>
<p><span class="popup" onclick="myFunction('myPopup2')">Click me to toggle the second popup!<span class="popuptext" id="myPopup2">Popup 2</span></span></p>
<p><span class="popup" onclick="myFunction('myPopup3')">Click me to toggle the third popup!<span class="popuptext" id="myPopup3">Popup 3</span></span></p>
<p><span class="popup" onclick="myFunction('myPopup4')">Click me to toggle the fourth popup!<span class="popuptext" id="myPopup4">Popup 4</span></span></p>

Everything works fine when I test it at JSFiddle, but on SquareSpace the popups don't disappear when you click on a different one. I assume I'm missing something that SquareSpace needs, but I'm not sure what.

Can anybody help?

PS: The original code was found here.