[flexcoders] Cannot access a property or method of a null object reference?

[xaero]

Hi
I'my working whit flex and amfphp.
In my exercise, the flex debug say this error:

TypeError: Error #1009: Cannot access a property or method of a null
object reference.
     at cys/changeHandler()[D:\My
Documents\FlexBuilder3\cys\src\cys.mxml:51]
     at cys/__atclass_close()[D:\My
Documents\FlexBuilder3\cys\src\cys.mxml:70]

line around 51:

public function changeHandler(event:Event):void
             {
                 var atClass:Object = new Object;
                 atClass.atcls =
ComboBox(event.target).selectedItem.data;

                 gateway.call("Cys.getStudents", new Responder(getResult,
onFault), atClass); // this is line 51
             }

getResult is another function that give the result to ComboBox's
dataProvider:
public function getResult(result:Array):void
             {
                 ssProvider = result;
             }

onFault:
public function onFault(fault:String ):void
             {
                 trace(fault);
             }

line 70:
<mx:ComboBox y="202" id="atclass" close="changeHandler(event);"
dataProvider="{atClassProvider}" fontSize="12" x="40"></mx:ComboBox>
this comboBox can display properly.

Cys is a Class, it is under amfphp/services  and its file code:
<?php
Define('DATABASE_SERVER', 'localhost');
Define('DATABASE_USERNAME', 'root');
Define('DATABASE_PASSWORD', '123456');
Define('DATABASE_NAME', 'cys');

class Cys
{
     var $mysqli;

     function Cys()
     {
         # Connect to MySQL database....
         $this->mysqli = new mysqli(DATABASE_SERVER, DATABASE_USERNAME,
DATABASE_PASSWORD, DATABASE_NAME);
         # Check MySQL connection
         if (mysqli_connect_errno()) {
             # Dont use die (Fatal Error), return useful info to the
client
             trigger_error("AMFPHP Remoting class could not connect: " .
mysqli_connect_error());
         }
     }
     function getStudents($atClass) {
         # Return a list of all the users
         $atcls =
$this->mysqli->real_escape_string(trim($atClass['atcls']));
         $sql = "SELECT * from students where atclass = '".$atcls."'";
         if ([EMAIL PROTECTED]>mysqli->query($sql)) {
             $errno=$this->mysqli->errno;
             $this->mysqli->close();
             trigger_error("AMFPHP Remoting class database SELECT query
error: " . $errno);
         }
         while ($row = $result->fetch_assoc()) {
             $user_array[] = $row;
         }
         return($user_array);
     }
}
?>