Get the treeNode's parent node.
Do not initialize or modify it, it is created by the zTree.
JSON data object of treeNode's parent node.
If treeNode is root, return null.
var treeObj = $.fn.zTree.getZTreeObj("tree");
var sNodes = treeObj.getSelectedNodes();
if (sNodes.length > 0) {
var node = sNodes[0].getParentNode();
}