树形图tvPublUI中的节点是根据数据表动态生成的，现在想删除checkbox选中的节点，代码如下：

foreach (TreeNode nNode1 in tvPublUI.Nodes)
                {
                    foreach (TreeNode nNode2 in nNode1.Nodes)
                    {                       
                        if (nNode2.Checked == true)
                        {                           
                            DataRow nRw = nNode2.Tag as DataRow;
                            nRw["bAbleCtrl"] = 1;
                            nNode2.Remove();
                        }
                    }
                }
运行之后出现如下错误提示：未将对象引用设置到对象的实例
求高手帮忙！！！