Fix to sort nested groups - Related to #2810 (#2817)

lib/timeline/component/ItemSet.js

@@ -1157,7 +1157,8 @@ ItemSet.prototype._orderNestedGroups = function(groupIds) {
       var nestedGroups = this.groupsData.get({
         filter: function(nestedGroup) {
           return nestedGroup.nestedInGroup == groupId;
-        }
+        },
+        order: this.options.groupOrder
       });
       var nestedGroupIds = nestedGroups.map(function(nestedGroup) { return nestedGroup.id })
       newGroupIdsOrder = newGroupIdsOrder.concat(nestedGroupIds);