[]调用Laya.Utils.parseXMLFromString 时一直报undefined的错误

function treeInit() {
    let treeData = "<data>";
    for (let i = 0; i < 5; ++i) {
        treeData += "<item label='Directory " + (i + 1) + "' isOpen='true'>";
        for (let j = 0; j < 5; ++j) {
            treeData += "<leaf label='File " + (j + 1) + "'/>";
        }
        treeData += "</item>";
    }
    treeData += "</data>"; 
    let xml = Utils.parseXMLFromString(treeData); //调用这一句一直是undefined
    // let tree = new Laya.Tree();
    // tree.scrollBarSkin = "";
    // tree.itemRender = initTreeItem();
    // tree.xml = treeData;
    // tree.size(300, 300)
    // tree.x = (Laya.stage.width - tree.width) / 2;
    // tree.y = (Laya.stage.height - tree.height) / 2;
    // Laya.stage.addChild(tree);
}