Adding a block into Sonata Admin Interface

I am trying to add a block into my admin dashboard, using sonataadminbundle. I tried to follow the directives written here in the first answer : How to add custom link or button to SonataAdminBundle Dashboard in Symfony2

But I have a problem in my BlockService class.

The code is as follow :

namespace AppBundle\Block;

use Symfony\Component\HttpFoundation\Response;
use Sonata\AdminBundle\Form\FormMapper;
use Sonata\CoreBundle\Validator\ErrorElement;
use Sonata\BlockBundle\Model\BlockInterface;
use Sonata\BlockBundle\Block\BaseBlockService;

class InvitationBlockService extends BaseBlockService
{
    public function getName()
    {
        return 'Invitation manager';
    }

    public function getDefaultSettings()
{
    return array();
}

public function validateBlock(ErrorElement $errorElement, BlockInterface $block)
{
}

public function buildEditForm(FormMapper $formMapper, BlockInterface $block)
{
}

public function execute(BlockInterface $block, Response $response = null)
{
    // merge settings
    $settings = array_merge($this->getDefaultSettings(), $block->getSettings());

    return $this->renderResponse('InstitutoStoricoNewsletterBundle:Block:block_invitation.html.twig', array(
        'block'     => $block,
        'settings'  => $settings
    ), $response);
}

}

But I have a compilation error on the validatBlock, buildEdittForm and execute method :

Declaration must be compatible with BlockServiceInterface->validateBlock(errorElement : \Sonata\AdminBundle>\Validator\ErrorElement, block : \Sonata\BlockBundle\Model\BlockInterface) Class hierarchy checks: abstract methods implementation, implementing/overriding method is compatibility with super declaration. All violations result in PHP fatal errors. It's not recommended to disable or suppress this inspection.

Do you have any idea ?

Thank you for reading.

I finally found the cause of my problem : I had to self-update the composer.

It looks like my composer was to old, and did not fetch every files or dependancy I needed.

So, composer self-update saved me.