Symfony 2.7: Call to a member function get() on null

Question

I have the following controller in a Symfony 2.7 application:

namespace MyCompany\AppBundle\Controller;

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;

class PdfPrevewController extends Controller
{

    public function __construct()
    {
        //TODO: Inject the em instead of extending controller.
        //      (An error resulted when attempting to do that.)

        $this->em = $this->get('doctrine.orm.default_entity_manager');
    }

    /**
     * @Route("/admin/pdf-preview/by-document-id/{id}", name="pdf_preview_by_document_id")
     */
    public function createPdfPreviewAction($id = 0)
    {
        die('Started.');
    }
}

And when I bring up the controller in a browser, I get the following message:

Error: Call to a member function get() on null

... which I don't really understand, since extending the controller class normally gives access to the container. What am I missing here?

====

Update: I also tried defining my controller as a service and setting the container there:

  app.controller.pdf_preview:
    class: Exozet\AppBundle\Controller\PdfPreviewController
    calls:
      - [setContainer, ['@service_container']]

... with no luck. The same error message still shows up.

Answer 1

    $this->em = $this->get('doctrine.orm.default_entity_manager');

isn't available in constructor yet ... instead use dependency injection to set your entitymanager (clean approach)

public function __construct(EntityManagerInterface $entityManager)

{

    $this->em = $entityManager;
}

or call the service in another function later (when all services are already set up)