Fix graph2D render issue (#2470)

lib/timeline/Core.js

@@ -124,7 +124,7 @@ Core.prototype._create = function (container) {
   this._redraw = util.throttle(this._origRedraw);
   
   this.on('_change', function (properties) {
-    if (me.itemSet.initialItemSetDrawn && properties && properties.queue == true) {
+    if (me.itemSet && me.itemSet.initialItemSetDrawn && properties && properties.queue == true) {
       me._redraw()
     } else {
       me._origRedraw();