Symfony3 form: View transformer error suggests using view transfomer - php

Question

I wanted to create a simple HiddenEntityType, so that I can have hidden form fields that represent an entity. The simplest solution by far looked to be to employ a (view) transformer.
However I cannot see to get it to work.
Based on documentation here
...the forward transform needs to return something you can put into HTML, a string(e.g. the id of my entity) and the reverseTransform must transform an id into an entity.
With that in mind here is the simple class I made:
namespace AppBundle\Form\Type;
use Doctrine\ORM\EntityManagerInterface;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\CallbackTransformer;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
class HiddenEntityType extends AbstractType
{
/**
* #var EntityManagerInterface
*/
private $entityManager;
/**
* Constructor
* #param EntityManagerInterface $entityManager
*/
public function __construct(EntityManagerInterface $entityManager)
{
$this->entityManager = $entityManager;
}
/**
* #inheritDoc
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$repository = $this->entityManager->getRepository($builder->getDataClass());
$builder->addViewTransformer(new CallbackTransformer(
function ($entity) use($repository) { //Forward
dump('Forward');
dump($entity);
if (!is_null($entity)) return $entity->getId();
return $entity;
},
function ($id) use($repository) { //Reverse
dump('Reverse');
dump($id);
return $repository->find($id);
}
));
}
/**
* #inheritDoc
*/
public function getParent()
{
return 'hidden';
}
/**
* {#inheritdoc}
*/
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults([
'data_class' => null,
]);
}
}
I receive the following illogical error:
An exception has been thrown during the rendering of a template ("The form's view data is expected to be an instance of class AppBundle\Entity\MyEntity, but is a(n) integer. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms a(n) integer to an instance of AppBundle\Entity\MyEntity.").
When creating the form, based on the dumps you see above, the forward transform is called twice - once with null and once with the AppBundle\Entity\MyEntity as its argument.
What might I be doing wrong?
Sy 3.3.6
PHP 7.1.8