Javascript SpaceTree如何在节点上添加子树单击

我已经编写了以下显示错误的代码：

无法读取null的属性“children”

我希望在单击节点时加载子节点

request:function(nodeId, level, onComplete)
            {
                alert(nodeId+'p   '+level+' p  '+onComplete);
                var i=0;
              $.ajax({
                    url: '/users/userhierarchy.json?id=' + nodeId,
                   dataType: 'json',
                   success: function(json){
                        var tree = json;
                        var subtree = $jit.json.getSubtree(tree, nodeId);

                        $jit.json.prune(subtree, level);
                        i++;
                        ans= {
                            'id': nodeId,
                            'children': subtree.children
                        };
                        onComplete.onComplete(nodeId, ans);

                    },

                });

            }