Bug fix for null parent (#3342)

lib/timeline/component/item/Item.js

@@ -64,7 +64,10 @@ Item.prototype.setData = function(data) {
   if (groupChanged && this.parent != null) {
     this.parent.itemSet._moveToGroup(this, data.group);
   }
-  this.parent.stackDirty = true;
+  
+  if (this.parent) {
+    this.parent.stackDirty = true;
+  }
   
   var subGroupChanged = data.subgroup != undefined && this.data.subgroup != data.subgroup;
   if (subGroupChanged && this.parent != null) {