Small bugfix to prevent null pointer exception (#2116)

lib/timeline/component/ItemSet.js

@@ -880,7 +880,7 @@ ItemSet.prototype._onUpdate = function(ids) {
   ids.forEach(function (id) {
     var itemData = me.itemsData.get(id, me.itemOptions);
     var item = me.items[id];
-    var type = me._getType(itemData);
+    var type = itemData ? me._getType(itemData) : null;
 
     var constructor = ItemSet.types[type];
     var selected;
@@ -898,7 +898,7 @@ ItemSet.prototype._onUpdate = function(ids) {
       }
     }
 
-    if (!item) {
+    if (!item && itemData) {
       // create item
       if (constructor) {
         item = new constructor(itemData, me.conversion, me.options);