OrderItem attaching as Collection to Order problem

AdminOrderCreateType.php

final class AdminOrderCreateType extends AbstractResourceType {
...
$builder
            ->add('items', CollectionType::class, [
                'entry_type' => AdminOrderItemCollectionType::class,
                'allow_add' => true,
                'allow_delete' => true,
                'by_reference' => false,
                'block_name' => 'entry'
            ])
...

AdminOrderItemCollectionType

class AdminOrderItemCollectionType extends AbstractResourceType {

    /**
     * @var RepositoryInterface
     */
    private $productRepository;

    public function __construct($dataClass, $validationGroups, RepositoryInterface $productRepository) {
        parent::__construct($dataClass, $validationGroups);
        $this->productRepository = $productRepository;
    }

    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $product = $this->productRepository->findOneByCode("ogu1");
        $builder
            ->add('product', ProductChoiceType::class, [
                'mapped' => false
            ]);
            /*->addEventListener(FormEvents::PRE_SUBMIT, function(FormEvent $event) use ($builder, $options){


            })*/
            $builder->add('variant', AdminOrderItemType::class, [
                'product' => $product
            ]);
        ;
    }}

services.yml

app.form.admin.order_item.collection.create:
        class: AppBundle\Form\AdminOrderItemCollectionType
        arguments:
            - %sylius.model.order_item.class%
            - %sylius.form.type.checkout_address.validation_groups%
            - '@sylius.repository.product'
        tags:
            - { name: form.type }