Simple Testimonials page problem

<?php
class Testimonial extends DataObject {

   public static $db = array(
      'Name' => 'Varchar(255)',
      'Testimonial' => 'Varchar(255)'
   );
   
   public static $has_one = array(
      'TestimonialsPage' => 'TestimonialsPage'
   );
   
   public function getCMSFields() {
          $fields = parent::getCMSFields();
         $fields->removeFieldFromTab("Root.Main","TestimonialsPageID");
         return $fields;      
    }

}

<?php
class TestimonialsPage extends Page {

   public static $has_many = array(
      'Testimonials' => 'Testimonial' // Link the Testimonial Items
   );
   
   public function getCMSFields() {
      $fields = parent::getCMSFields();
      
       // Create a default configuration for the new GridField, allowing record editing
       $config = GridFieldConfig_RecordEditor::create();
       $config->getComponentByType('GridFieldDataColumns')->setDisplayFields(array(
          'Name' => 'Name',
                  'Testimonial' => 'Testimonial'
       ));
       // Create a gridfield to hold the student relationship
       $testimonialsField = new GridField(
       'Testimonial', // Field name
       'Testimonials', // Field title
       $this->Testimonials(), // List of all related students
       $config
       );
       // Create a tab named "Students" and add our field to it
       $fields->addFieldToTab('Root.Testimonials', $testimonialsField);
       return $fields;
   }

}
class TestimonialsPage_Controller extends Page_Controller {

}