APEX

I am using Apex 4.0.2. My goal is to have my success message flash at the top after I succesfully fill out a form. I have a javascript function:

<script type="text/javascript">
{var i = 1,timer;
window.onload=function() {
  　timer = setInterval('flash()', 500);
}
function flash() {
 if (i<10000) {
        if (i%2 == 0) { 
              document.getElementById('flash').style.backgroundColor = '#ffffff';
         } else {
              document.getElementById('flash').style.backgroundColor = '#ffff00';
         }
 } else {
      document.getElementById('flash').style.backgroundColor = '#ffffff';
      clearInterval(timer);
  }
 i++;
}
</script>

I placed this code originally on the javascript tab of the page that the form branches to. Later I moved it to the process success message along with the call to the function

<center>
<table id="flash"  BORDER=0 >
<tr>
<td>Success!</td>
</tr>   
</table>
</center>

I receive an error from the page on Firebug

document.getElementById("flash") is null
document.getElementById('flash').style.backgroundColor = '#ffff00';

Internet Explorer tells me :
Message: Object required


I think my problem is that the success message region object does not always exist. Is there any way to execute the function only when I make that call to display the success message?

In advance,
Thanks so much for your help!