private void TreeView_DragOver(object sender, DragEventArgs e)
        {
            if (Handler == null)
            {
                return;
            }

            e.Effect = DragDropEffects.None;

            if (draggedNodes != null && UI.TreeView.DropPosition.Node != null)
            {
                TreeNodeAdv dropTarget = UI.TreeView.DropPosition.Node;

                if (TreeModel.CanDrop(draggedNodes, dropTarget, UI.TreeView.DropPosition.Position))
                {
                    e.Effect = DragDropEffects.Move;
                }
            }
        }