getElementById breaks script

I'm working on a little project, but I'm stuck and don't know what I'm doing wrong.

I use Ajax to call a php-script that updates a MYSQL-row.

The javascript:

function savecust()
{
if (window.XMLHttpRequest)
  {
  xmlhttp=new XMLHttpRequest();
  }
else
  {
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("custdetails").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","./ajax/savecust.php?id=" + document.getElementById('id').value,true); //More arguments should still be passed.
xmlhttp.send();
}

If I remove the document.getElementById('id').value the script works and does what is is supposed to do. (I get the response from the PHP, obviously

I load the script in the head of the page along with some other scripts:

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

This is how I call the script:

<input type='button' onclick='savecust()' value='Save' />

Any help would be greatly appreciated ;-).

Thanks!

EDIT

• The div where the content should go, exists.
• The PHP-scripts returns an error for now (because it doesn't get all the required parameters)
• The content (and textfield with id: 'id') is loaded by another script. Is this a problem? If you look at the source in a browser the textfields are not there. They are however visible in the browser.

EDIT 2

So, I ran this test on top of my script:

var thisid = 'id';//should get info from <input type='text' name='id' />
var T = document.getElementById(thisid);
if (T) {
     alert("element exists" + T.val开发者_开发技巧ue);
} else {
alert("nothing to be found");
}

It is indeed unable to find the element. This pointed me in the right direction.

<input type='text' name='id' />is inserted by another Ajax-script. So I think the second script is unable to find it.

How do I solve this?

You cannot access an element property if it does not exist.

Perhaps a test:

var thisid = 'id';//whatever it REALLY is...
    var T = document.getElementById(thisid);
    if (T) {
         alert("element exists" + T.value);
    }

The most obvious reason is that there is no field whose id is id or the HTML has not been fully loaded when the script runs.

You should check your browser's script errors (use Firebug with Firefox or press F12 on Internet Explorer)