Symfony 3.4 autowire service
As noted by @Cerad in the comments, you should change EntityManager to EntityManagerInterface in your constructor.
Change the line
use Doctrine\ORM\EntityManager;to
use Doctrine\ORM\EntityManagerInterface;And also change the line
public function __construct(FormFactoryInterface $formFactory, EntityManager $em, RouterInterface $router)to
public function __construct(FormFactoryInterface $formFactory, EntityManagerInterface $em, RouterInterface $router)
The Doctrine\Common\Persistence\ObjectManager interface is no longer aliased to the doctrine.orm.entity_manager service, use Doctrine\ORM\EntityManagerInterface instead.