Add a draggable item to tree will cause js error

<zk>
	<button label="add item">
		<attribute name="onClick"><![CDATA[
			Treeitem item = new Treeitem("new");
			item.setDraggable("true");
			tc.appendChild(item);
		]]></attribute>
	</button>
	<tree>
		<treechildren>
			<treeitem label="item 1" open="false">
				<treechildren id="tc">
				
				</treechildren>
			</treeitem>
		</treechildren>
	</tree>
</zk>