Dynamically add Elements to Symfony Form Collection without Data

RecordResultForm.php

<?php

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\CollectionType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;

final class RecordResult extends AbstractType
{
    private $contractRepository;

    public function __construct($contractRepository) {
        $this->contractRepository = $contractRepository;
    }

    /**
    * @param FormBuilderInterface $builder
    * @param array $options
    */
	public function buildForm(FormBuilderInterface $builder, array $options)
	{
    	$builder->add('customFields', CollectionType::class, []);

    	$builder->addEventListener(
        	FormEvents::POST_SET_DATA,
	        function (FormEvent $event) {
	            $data = $event->getData();
	            $form = $event->getForm();

	            $contract = $this->contractRepository->find($data->contractId());

                foreach ($contract->customFields() as $customField) {
                    $form
                        ->get('customFields')
                        ->add($customField->getName(), TextType::class, [
                            'label' => $customField->getLabel()
                        ])
                    ;
                }
            }
        );
	}
}