怎么解决Cannot read property 'getElementsByTagName' of null

<%@ page language="java" contentType="text/html; charset=UTF-8"

	pageEncoding="UTF-8"%>

<%@ page import="entity.*,java.util.List,dao.impl.*"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

<script type="text/javascript" src="js/JSAjax.js"></script>

<script type="text/javascript" src="js/regist.js"></script>

<script type="text/javascript">

	function showZy(){

		var zys;

		if(xmlHttp.readyState==4){

			if(xmlHttp.status==200){

				zys=xmlHttp.responseXML.getElementsByTagName("zy");

				while(document.all.zy.options.length>0){

					 document.all.zy.removeChild(document.all.zy.childNodes[0]);

				}

			}

			for(var i=0;i<zys.length;i++){

				var option=document.createElement("OPTION");

				option.text=zys[i].getElementsByTagName("zyname")[0].firstChild.data;

				option.value=zys[i].getElementsByTagName("zyvalue")[0].firstChild.data;

				document.all.zy.options.add(option);

			}

		}

	}

	function select() {

		var p = document.getElementById("district").value;

		createXMLHttpRequest();

		xmlHttp.onreadystatechange = showZy;

		xmlHttp.open("POST", "ZyServlet", true);

		xmlHttp.setRequestHeader("Content-type",

				"application/x-www-form-urlencoded");

		xmlHttp.send("xy=" + p);

	}

</script>

package servlet;



import java.io.IOException;

import java.io.PrintWriter;

import java.sql.ResultSet;

import java.sql.SQLException;



import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;



import util.DBUtil;



/**

 * Servlet implementation class ZyServlet

 */

@WebServlet("/ZyServlet")

public class ZyServlet extends HttpServlet {

	private static final long serialVersionUID = 1L;

       

    /**

     * @see HttpServlet#HttpServlet()

     */

    public ZyServlet() {

        super();

        // TODO Auto-generated constructor stub

    }



	/**

	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

	 */

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

		// TODO Auto-generated method stub

		response.getWriter().append("Served at: ").append(request.getContextPath());

		response.setContentType("text/xml;charset=utf-8");

		String xyid=request.getParameter("xy");

		PrintWriter out = response.getWriter();

		String SQL_SELECT="SELECT * FROM ZY";

		DBUtil db=new DBUtil();

		db.getConnection();

		String sql=SQL_SELECT+" where xyid=?";

		ResultSet rs = db.executeQuery(sql, new String[] {xyid});

		out.print("<response>");

		try {

			while(rs.next()){

				String a,b;

				a=rs.getString("id");

				b=rs.getString("zy");

				out.print("<zy>");

				out.print("<zyname>"+b+"</zyname>");

				out.print("<zyvalue>"+a+"</zyvalue>");

				out.print("</zy>");

			}

		} catch (SQLException e) {

			// TODO Auto-generated catch block

			e.printStackTrace();

		}

		out.print("</response>");

		out.flush();  

        out.close();

	}



	/**

	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

	 */

	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

		// TODO Auto-generated method stub

		doGet(request, response);

	}



}

百度到说问题是因为response.setContentType("text/xml;charset=utf-8");这个的问题 可是写上了还是不对 求解0.0