Symfony CollectionType of Entities

Question

I'm working on a project on Symfony 3.4 without Doctrine (we use a homemade ORM similar to Eloquent) and have been stucked trying to make a CollectionType of an Entity I use : IntroductionSlide.

The form looks like this :

You can add or delete IntroductionSlides and each IntroductionSlide is defined only by its image. As the image depends on the locale, it's handled by a TranslatableImageType which creates a field per locale. Clicking on the little flag allows you to choose target locale's input field.

Here's what my code looks like :

The creation of the CollectionType :

case 'introduction_slides':
    $options['label'] = "Pages de présentation";
    $options['entry_type'] = IntroductionSlideType::class;
    $options['allow_add'] = true;
    $options['allow_delete'] = true;
    $options['attr']['class'] = "introduction-slides";
    $options['required'] = false;
    $this->form->add($field, CollectionType::class, $options);
    break;

The IntroductionSlideType :

<?php

namespace AppBundle\Form\Type;

use AppBundle\Model\IntroductionSlide;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\FormView;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormBuilderInterface;

class IntroductionSlideType extends AbstractType
{
    public function buildView(FormView $view, FormInterface $form, array $options)
    {}

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $options['mapped'] = false;
        $builder->add("images", TranslatableImageType::class, $options);
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => IntroductionSlide::class,
        ));
    }
}

And The TranslatableImageType :

<?php

namespace AppBundle\Form\Type;

use AppBundle\EventSubscriber\StaticConfig;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\FileType;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\FormView;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Validator\Constraints\Image;

class TranslatableImageType extends AbstractType
{
    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        $view->vars['locales'] = $options['locales'];
        $view->vars['locale'] = $options['locale'];
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $file_options = array_merge($options, $options["field_options"]);

        unset($file_options['locales'], $file_options['locale'], $file_options["field_options"]); // not valid text options
        $file_options['compound'] = false; // sub items are simple widgets

        $file_options['constraints'] = [new Image([
            'mimeTypes' => ["image/png", "image/jpg", "image/jpeg", "image/gif"],
            'mimeTypesMessage' => "Seul un fichier avec l'extension png, jpg, jpeg ou gif peut être uploadé.",
            "maxSize" => "7M",
            'maxSizeMessage' => "Ce fichier est trop volumineux ({{ size }} {{ suffix }}). La limite de volume est de {{ limit }} {{ suffix }}.",
        ])];

        foreach ($options['locales'] as $locale) {
            $file_options['attr']['lang'] = $locale;
            $builder->add($locale, FileType::class, $file_options);
        }
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => null,
        ));

        $resolver->setDefaults(array(
            'locales' => StaticConfig::getLocales(),
            'locale' => StaticConfig::getLocale(),
            'field_options' => [],
        ));
    }

    public function getBlockPrefix()
    {
        return 'translatableImage';
    }
}

Here what I was thinking Symfony would do (with no data passed to the collection) :
1. define a CollectionType
2. define an IntroductionSlideType inside the CollectionType
3. instanciate an empty IntroductionSlide and use is as data for the IntroductionSlideType
4. create a field "images" to the builder and call $IntroductionSlide->getImages() to feed its value
5. define a TranslatableImageType inside the IntroductionSlideType
6. use $IntroductionSlide->getImages()'s returned value as data (contains an array of locale => image_name)

Instead, Symfony triggers an Exception :

Cannot read index "0" from object of type "AppBundle\Model\IntroductionSlide" because it doesn't implement \ArrayAccess.

I don't see why it would try to reach an index "0" in an IntroductionSlide.

Setting up a break-point showed me this happens when Symfony is trying to build the field "images" in IntroductionSlideType.

Adding $options['mapped'] = false; to $builder->add("images", TranslatableImageType::class, $options); makes it able to reach the view without triggering the Exception.
When submitting the form, after the data is processed by the Form component, here's what it looks like :