Ajax PHP MySQL cascading html elements

I am just starting to get to grips with Ajax as a way of providing asynchronous support for interaction with a MySQL database without resorting to form submission.

I am working on a project in which one of the pages has two selects and a textfield. These elements need to cascade. The first select is on the page to begin with, the second appears once a selection has been made. The textfield appears once  selection has been made in the second select. The first option on both selects is a null option inviting the user to make a selection. What I want is for the second select and textfield to disappear if the null option is subsequently selected on select 1, and for the textfield to disappear if the null option is selected on select 2 - i.e., graceful management of the cascading process.

I have four files I am using:
- the file containing the form and the code for querying the database to populate select 1 (mgtAddDocs.php)
- one php file containing the routine for querying the database to populate select 2 (getAdminGroups.php)
- one php file containing the routine for querying the database to populate the textfield (getAGPassword.php)
- one javascript file containing the ajax script (processSelections.js)

At the moment the mechanisms work up to the point at which, once initial selections have been made and all three elements are visible, I select the null option on one of the selects at which time the following errors appear:

Notice: Trying to get property of non-object in /Users/mike/Sites/projectRed/mgt/addDocs/getAdminGroups.php on line 9 [...or getAGPassword.php if select 2 null has been selected]

Fatal error: Call to a member function fetch_assoc() on a non-object in /Users/mike/Sites/projectRed/mgt/addDocs/getAdminGroups.php on line 9 [...or getAGPassword.php if select 2 null has been selected]

Presumably this has something to do with the $_GET array being empty when the null option is selected even though the javascript is still triggered by 'onchange' but I am not sure what's wrong and should really appreciate some assistance to sort the code out please.

<!-- Container file (mgtAddDocs.php) -->
<!-- js declaration in the head -->
<script type="text/javascript" src="processSelections.js"></script>
 
<!-- This is an excerpt from the container file body with the key parts of the table -->
<td class="twentyFive">
    <select name="selectDocZone" class="fullWidth" onchange="showAdminGroups(this.value)">
        <option>Select a document zone...</option>
	    <?php
	    do 
	    { ?>
	        <option value="<?php echo $row_rstDocumentZones['zone_pk']; ?>">
                <?php echo $row_rstDocumentZones['zoneTitle']; ?>
		</option>
	    <?php } while ($row_rstDocumentZones = mysql_fetch_assoc($rstDocumentZones)); ?>
    </select>
</td>
<td class="twenty" colspan="3"><div id="ag" name="ag"></div></td>
<td class="ten"><div id="agpw" name="agpw"></div></td>
<!-- 
--
php file to populate select 2 (getAdminGroups.php)
--
-->
<?php
// Get the zone number from the $_GET array
$z = $_GET['zone'];
	
// Create a database connection
@ $db = new mysqli('localhost', 'user', 'password', 'database');
if (!$db) die ('Could not connect to the database: '.mysql_error());
		
// Get the admin group relating to the zone value
$query = sprintf("SELECT adminGroup_pk, agMembers FROM adminGroup WHERE zoneA_id = %s OR zoneB_id = %s", $z, $z);
$result = $db->query($query);
$num_results = $result->num_rows;
		
echo '<select name="selectAdminGroup" class="fullWidth" onchange="showAGPassword(this.value)">'; 
echo '<option>Select an admin group...</option>';
for ($i = 0; $i < $num_results; $i++)
{ 
	$row = $result->fetch_assoc();
	echo '<option value="';
	echo $row['adminGroup_pk'].'"'; 
	echo '>';
	echo $row['agMembers'];
	echo '</option>';
}
echo '</select>';
	
// free up the resultset
$result->free();
?>
<!-- 
--
php file to populate textfield (getAGPassword.php)
--
-->
<?php
// Get the admin group number from the $_GET array
$ag = $_GET['ag'];
	
// Create a database connection
@ $db = new mysqli('localhost', 'user', 'password', 'database');
if (!$db) die ('Could not connect to the database: '.mysql_error());
		
// Get the ag password relating to the ag value
$query = sprintf("SELECT agPassword FROM adminGroup WHERE adminGroup_pk = %s", $ag);
$result = $db->query($query);
$num_results = $result->num_rows;	// There is only one
$row = $result->fetch_assoc();
		
// Create the textfield element
echo '<input type="text" class="agPassword" readonly="true" size="10" maxlength="10" value="'.$row['agPassword'].'" />';
		
// free up the resultset
$result->free();
?>
<!-- 
--
js file (processSelections.js)
--
-->
var xmlHttp;
 
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;
}
 
function zoneStateChanged() 
{ 
	if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
	{ 
		document.getElementById("ag").innerHTML=xmlHttp.responseText;
	} 
}
 
function agStateChanged() 
{ 
	if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
	{ 
		document.getElementById("agpw").innerHTML=xmlHttp.responseText;
	} 
}
 
function showAdminGroups(str)
{ 
	xmlHttp=GetXmlHttpObject();
	if (xmlHttp==null)
	{
		alert ("Browser does not support HTTP Request");
	 	return;
	}
	var url="getAdminGroups.php";
	url=url+"?zone="+str;
	url=url+"&sid="+Math.random();
	xmlHttp.onreadystatechange=zoneStateChanged;
	xmlHttp.open("GET",url,true);
	xmlHttp.send(null);
}
 
function showAGPassword(str)
{ 
	xmlHttp=GetXmlHttpObject();
	if (xmlHttp==null)
	{
		alert ("Browser does not support HTTP Request");
	 	return;
	}
	var url="getAGPassword.php";
	url=url+"?ag="+str;
	url=url+"&sid="+Math.random();
	xmlHttp.onreadystatechange=agStateChanged;
	xmlHttp.open("GET",url,true);
	xmlHttp.send(null);
}

Select allOpen in new window