first of all, hello all

i found this forum while running out of ideas and being extremely desperate to fixing a probably small javascript error in a script. the script is supposed to open a small form window that allows the user to input an email address and update it to proceed. the form item is initially unchecked, but as the user clicks it and enter his email address, it updates the value of the email address and the box becomes "checkable". the problem is that with both IE and firefox, the box doesn't close again, doesn't get checkable and basically doesn't work. in the firefox debugging console, I found the following error:

Error: document.getElementById(input_array[i]) is null
Source File: sell_item.php
Line: 1106
does anyone have an idea what could be wrong?

the part responsible in the javascript for this section is:

Code:
<script language="javascript">
	function pg_popup_open(id)
	{
		if (document.getElementById(id).style.display == 'block')
		{
			document.getElementById(id).style.display = 'none';			
		}
		else
		{
			document.getElementById(id).style.display = 'block';
		}
		return false;
	}	
	
	function pg_update_settings(id, input_array)
	{
		xmlHttp=GetXmlHttpObject();
		
		if (xmlHttp==null)
		{
			alert ("Browser does not support HTTP Request");
			return;
		}

		var url    = '' + 'ajax_files/direct_payment_box.php';
		var action    = url + '?id=' + id + '&user_id=' + 100001;

		var chk_disabled = false;
		for ( var i in input_array )
		{
    		action += '&' + input_array[i] + '=' + document.getElementById(input_array[i]).value;
    		
    		if (document.getElementById(input_array[i]).value == '')
    		{
    			chk_disabled = true;
    		}
		}		
		
		xmlHttp.onreadystatechange = function() { 
			if (xmlHttp.readyState == 4)
			{
				var response = xmlHttp.responseText;
			}
		};
		xmlHttp.open("GET", action, true);
		xmlHttp.send(null);		
		
		document.getElementById('checkbox_' + id).disabled = chk_disabled;
		if (chk_disabled == true)
		{
			document.getElementById('checkbox_' + id).checked = false;
		}
		document.getElementById(id).style.display = 'none';
		return false;
	}
</script>
and the html code that is supposed to pop up the box is:

Code:
<input type="checkbox" name="payment_gateway[]" id="checkbox_pg_paypal" value="1"  disabled>
<span class="contentfont"><a href="javascript:;" onclick="pg_popup_open('pg_paypal');">PayPal</a></span>
<script language="javascript"> var array_pg_paypal = new Array(); array_pg_paypal[0] = 'pg_paypal_email';</script>
<div class="smallfont"><b>PayPal Email Address</b><br>
<input type="text" name="pg_paypal_email" id="pg_paypal_email" value="" /></div>
<div align="right"><input type="button" value="Proceed" onclick="pg_update_settings('pg_paypal', array_pg_paypal);" /></div>
i understand this is asking a lot, but if someone has a spare second that is a javascript guru, i promise i will name my first born after you!! lol