(URGENT HELP) ResponseXML is null....

Hi guys...
Im very new to JavaScript programming.. Im doing a JSP page form validation... I downloaded this code but every time my responseXML object getting null....

Im not using any external XMl file here to load data.... but still its not working for me....

My JS file.....

function validate(inputValue, fieldID){
if (xmlHttp){
if (fieldID){

inputValue = encodeURIComponent(inputValue);
fieldID = encodeURIComponent(fieldID);

cache.push("inputValue=" + inputValue + "&fieldID=" + fieldID);
}

try{

if ((xmlHttp.readyState == 4 || xmlHttp.readyState == 0) && cache.length > 0){

var cacheEntry = cache.shift();

xmlHttp.open("POST", serverAddress, true);
xmlHttp.setRequestHeader("Content-Type","text/xml; charset=utf-8");
xmlHttp.onreadystatechange = handleRequestStateChange
xmlHttp.send(cacheEntry);
}
}catch (e){
displayError(e.toString());
}
}
}

function handleRequestStateChange(){
if (xmlHttp.readyState == 4){
if (xmlHttp.status == 200){
try{
readResponse();
}catch(e){
displayError("HERE AT METHOD CALL >"+e.toString());
}
}else{
displayError(xmlHttp.statusText);
}
}
}

function readResponse(){
var response = xmlHttp.responseText;

if (response.indexOf("ERRNO") >= 0 || response.indexOf("error:") >= 0 || response.length == 0)
throw(response.length == 0 ? "Server error." : response);

responseXml = xmlHttp.responseXML;

xmlDoc = responseXml.documentElement;
result = xmlDoc.getElementsByTagName("result")[0].firstChild.data;
fieldID = xmlDoc.getElementsByTagName("fieldid")[0].firstChild.data;

message = document.getElementById(fieldID + "Failed");

message.className = (result == "0") ? "error" : "hidden";

setTimeout("validate();", 250);
}

function setFocus(){
document.getElementById("txtUsername").focus();
}

Thanks in advance......

ask one of the moderators to move the thread in this forum http://www.codingforums.com/forumdisplay.php?f=54 you have bigger chances to get a answer there.
Also put your code between [code ] and [/code ] tags.( remove spaces)

best regards and happy new year