fix failing test from ItemSet.js (#4256)

lib/timeline/component/ItemSet.js

@@ -541,6 +541,7 @@ ItemSet.prototype.show = function() {
 
 /**
  * Activates the popup timer to show the given popup after a fixed time.
+ * @param {any} popup
  */
 ItemSet.prototype.setPopupTimer = function (popup) {
     this.clearPopupTimer();