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