Joomla component Development: Invalid controller: name='', format='' - joomla

Question

I am trying out to develop components from here. I am getting an error on the admin section
500 - An error has occurred.
Invalid controller: name='', format=''
How to debug this? I dont even know what code is relevant to post.
File: admin/controller.php
// No direct access to this file
defined('_JEXEC') or die('Restricted access');
// import Joomla controller library
jimport('joomla.application.component.controller');
class TestimonialsController extends JController {
function display($cachable = false) {
// set default view if not set
JRequest::setVar('view', JRequest::getCmd('view', 'Testimonials'));
// call parent behavior
parent::display($cachable);
}
}
File: admin/testimonials.php
<?php
// No direct access to this file
defined('_JEXEC') or die('Restricted access');
jimport('joomla.application.component.controller');
$controller = JController::getInstance('Testimonials');
$controller->execute(JRequest::getCmd('task'));
$controller->redirect();
File: admin/views/testimonials/view.html.php
<?php
// No direct access to this file
defined('_JEXEC') or die('Restricted access');
// import Joomla view library
jimport('joomla.application.component.view');
class TestimonialsViewTestimonials extends JView {
function display($tpl = null) {
$items = $this -> get("Items");
$pagination = $this -> get("Pagination");
//Check for errors
if (count($errors = $this->get('Errors')))
{
JError::raiseError(500, implode('<br />', $errors));
return false;
}
// Assign data to the view
$this -> items = $items;
$this -> pagination = $pagination;
// Display the template
parent::display($tpl);
}
}

Answer 1

#mrN: What about the xml file? You can verify the sections <files> have all archives.
Example of error 500 when a file is missing:
<!-- file testimonials.xml -->
<!-- ERROR 500 because <em>admin/controller.php</em> is not installed -->
...
<administration>
<!-- Administration Menu Section -->
<menu>Testimonials</menu>
<!-- Administration Main File Copy Section -->
<!-- Note the folder attribute: This attribute describes the folder
to copy FROM in the package to install therefore files copied
in this section are copied from /admin/ in the package -->
<files folder="admin">
<!-- Admin Main File Copy Section -->
<filename>index.html</filename>
<filename>testimonials.php</filename>
<!-- SQL files section -->
<folder>sql</folder>
</files>
</administration>
...