这个错误如何解决？

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:372)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause java.lang.NullPointerException
org.apache.jsp.checkUserName_jsp._jspService(checkUserName_jsp.java:85)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.代码如下：
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<jsp:useBean id="connDB" scope="page" class="test.connDB"/>
<jsp:useBean id="chStr" scope="page" class="beans.chStr"/>
<%
String User_Name=chStr.chStr(request.getParameter("User_Name"));
ResultSet rs=connDB.executeQuery("select * from User_Table where User_Name='"+User_Name+"'");
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>检测用户名</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<link href="CSS/style.css" rel="stylesheet">
</head><body>
      <table width="90%"  border="0" cellspacing="0" cellpadding="0">
        <tr>
          <td height="7" align="center">&nbsp;</td>
        </tr>   
  <%
  if(rs.next()){
  %>        <tr>
          <td height="45" align="center"><%out.println("很报歉!<br><br>["+User_Name+"]用户名已经被注册!");%></td>
        </tr>
<%}else{%>
        <tr>
          <td height="56" align="center"><%out.println("祝贺您!<br><br>["+User_Name+"]用户名没有被注册!");%></td>
        </tr>
<%}%>
    </table>
</body>
</html>