JSP操作XML，删除节点问题

程序：
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();

path = application.getRealPath("/");

Document doc = db.parse(path+"left\\XML\\top.xml");
NodeList nl = doc.getElementsByTagName("Tree"); 
nl.item(0).removeChild(doc.getElementById("msdn"));报错：
HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:373)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause java.lang.NullPointerException
org.apache.xerces.dom.ParentNode.internalInsertBefore(ParentNode.java:339)
org.apache.xerces.dom.ParentNode.insertBefore(ParentNode.java:326)
org.apache.xerces.dom.NodeImpl.appendChild(NodeImpl.java:224)
org.apache.jsp.left.XML.deeptree_jsp._jspService(org.apache.jsp.left.XML.deeptree_jsp:99)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:99)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:325)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)