Chyba JavaScriptu - document.getElementById má hodnotu null

var xmlHttp

 function showUser(auction_id)
{ 
    xmlHttp=GetXmlHttpObject()
        if (xmlHttp==null)
            {
                alert ("Browser does not support HTTP Request")
                return
            }
            
 var url="rotate.php?auction_id="+auction_id

     xmlHttp.onreadystatechange=stateChanged 
    xmlHttp.open("GET",url,true)
    xmlHttp.send(null)
}

 function stateChanged() 
    { 
        if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
            { 
                var update = new Array();
                if(xmlHttp.responseText.indexOf('||' != -1)) 
                    {
                        update = xmlHttp.responseText.split('||');
      
                         var half= update.length /2;
        
                         for (i=0;i<=update.length-1 ;i++)
                            {
                                if(i<half){
                                    
                                     document.getElementById(update[i]).innerHTML = update[half+i];
                                }
                     
                             }
                    }
            } 
    }

 function GetXmlHttpObject()
    {
        var xmlHttp=null;
        try
            {
                // Firefox, Opera 8.0+, Safari
                xmlHttp=new XMLHttpRequest();
            }
    catch (e)
        {
         //Internet Explorer
         try
              {
                  xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
              }
    catch (e)
        {
              xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
          }
    }
return xmlHttp;
}

<?php
if(empty($_SERVER['HTTP_REFERER'])) exit;
session_start();


 include('includes/global.php');
include('language/english/site.lang.php');

 $item= new item();


 $auction_id=htmlentities($_GET["auction_id"], ENT_QUOTES);

 $id_array = explode(",",$auction_id);
$number = count($id_array);


 $values=mysql_query("SELECT A.auction_id, A.start_price,  A.currency, A.name, A.max_bid, A.end_time, AM.media_url
        FROM " . DB_PREFIX . "auctions A
        LEFT JOIN " . DB_PREFIX . "auction_media AM ON AM.auction_id=A.auction_id
        WHERE
        A.closed=0 AND A.active=1 AND A.approved=1 AND A.deleted=0 AND A.creation_in_progress=0 AND
        A.list_in!='store' AND A.hpfeat=1
        GROUP BY A.auction_id 
        ORDER BY RAND() LIMIT 0," . $number);
        

        
 foreach($id_array as $value){
    $spans.='featured_picture_'.$value.'||featured_title_'.$value.'||featured_details_'.$value.'||';
}

 while($item_details = mysql_fetch_array($values)){

     $auction_link = process_link('auction_details', array('name' => $item_details['name'], 'auction_id' => $item_details['auction_id']));
    
     $image_link = '<a href="'.$auction_link.'"><img src="'.((!empty($item_details["media_url"])) ? 'thumbnail.php?pic=' . $item_details["media_url"] . '&w=' . $layout['hpfeat_width'] . '&sq=Y' : 'themes/' . $setts['default_theme'] . '/img/system/noimg.gif').'" border="0" alt="'.$item_details['name'].'"></a>';
    

     $details = 
    "<b>".MSG_START_BID.":</b> ".$fees->display_amount($item_details['start_price'], $item_details['currency'])."<br>".
    "<b>".MSG_CURRENT_BID.":</b> ".$fees->display_amount($item_details['max_bid'], $item_details['currency'])."<br>".
    "<b>".MSG_ENDS.":</b> ".show_date($item_details['end_time']); 
    
     $spans.=$image_link.'||<a href="'.$auction_link.'">'.$item_details["name"].'</a>||'.$details.'||';
}

 print substr($spans,'',-2);
?>

<script type="text/javascript" language="Javascript" src="rotate.js"></script>

 <script language="JavaScript">
function repeat() {


         timer=setTimeout("repeat()",<?=$layout['hpfeat_rotate'];?>);
        showUser(document.getElementById('feat_auc').value);    
}
</script> 

<?
$value = implode(",",$auction_ids);
?>
<input type='hidden' value='<?=$value;?>' name='feat_auctions' id='feat_auc'/>

<? if ($layout['hpfeat_nb'] && $layout['hpfeat_rotate']>0) { ?>
    <script language="JavaScript">
    repeat();
    </script>
<? }?>