Fatal error: Call to a member function isComposite() on a non-object in /home/psmrepsc/public_html/sapphire/forms/FieldSet.php on line 178

I am getting the above error when checking out.

Here is my config:

<?php

global $project;
$project = 'mysite';

global $databaseConfig;
$databaseConfig = array(
   "type" => "MySQLDatabase",
   "server" => "localhost",
   "username" => "xxxxxxxx",
   "password" => "xxxxxxxxxxx",
   "database" => "xxxxxxxxx",
);
// Sites running on the following servers will be
// run in development mode. See
// http://doc.silverstripe.com/doku.php?id=devmode
// for a description of what dev mode does.
Director::set_dev_servers(array(
   'localhost',
'xxxxxxx',

));

// This line set's the current theme. More themes can be
// downloaded from http://www.silverstripe.com/themes/
SSViewer::set_theme('psm2009');

// Define which payment methods your site uses.
Payment::set_supported_methods(array(
   'PayPalPayment' => 'Credit card (via PayPal)',
   'ChequePayment' => 'Business Check (must clear)'
));

// Set the email of the admin who looks after receiving the order emails.
Order::set_email('xxxxxxxxxxx@xxxxx');