private void AddTreeViewItem(ItemCollection items, WorkItem workItem)
        {
            bool firstChild = (items.Count == 0);
            WorkItemTreeViewItem item = new WorkItemTreeViewItem(workItem);
            items.Add(item);

            if (firstChild && (item.Parent is WorkItemTreeViewItem))
            {
                (item.Parent as WorkItemTreeViewItem).IsExpanded = true;
            }
        }