Please help to resolve following issue

i unable to create page, i got following error :

Error at line 137 of C:\xammp\htdocs\aac-uk\trunk\application\live\framework\forms\gridfield\GridField.php

My Code is :

<?php
class TestimonialPage extends Page
{
private static $has_one = array ('Testimonials' => 'Testimonial');

public function getCMSFields()
{
$fields = parent::getCMSFields();
// Create a default configuration for the new GridField, allowing record editing
$config = GridFieldConfig_RelationEditor::create();
// Set the names and data for our gridfield columns
$config->getComponentByType('GridFieldDataColumns')->setDisplayFields(array(
'Photo' => 'Photo',
'TestimonialPage'=> 'TestimonialPage' // Retrieve from a has-one relationship
));
// Create a gridfield to hold the Testimonial relationship
$testimonialsField = new GridField(
'Testimonials', // Field name
'Testimonial', // Field title
$this->Testimonials(), // List of all related Testimonial
$config
);
// Create a tab named "Testimonial" and add our field to it
$fields->addFieldToTab('Root.Testimonial',$testimonialsField);
return $fields;

}

}

class TestimonialPage_Controller extends Page_Controller
{

}

?>