Symfony 4 map disabling not working - php

Question

I'm trying to put two different entities (employee and contractor) in one selectbox (EntityType). So I created an unmapped entity called Receiver that puts all the loaded entities in one collection.
When debugging everything goes well, but as soon as it creates the form it gives me the following exception:
Class "App\Receiver\Receiver" seems not to be a managed Doctrine entity. Did you forget to map it?
And it indeed is not a managed Doctrine entity because it should only exist in the code. So in my type I've added the mapped => false option, however it still gives me that same exception.
Here is some code:
OrderController.php where the overall form is being built and a collection type is made
->add('orderNotes', CollectionType::class, [
'entry_type' => NotesType::class,
'allow_add' => true,
'allow_delete' => true,
'prototype' => true,
'by_reference' => false,
'required' => false,
'label' => false,
'mapped' => false
])
And here is the NotesType Form
public function buildForm(FormBuilderInterface $builder, array $options)
{
$options['mapped'] = false;
$data = array_merge($data, Receiver::loadData($this->entityManager->getRepository("App:Contractor")->findAll(), "name", "id"));
$data = array_merge($data, Receiver::loadData($this->entityManager->getRepository("App:Employee")->findAll(), "name", "id"));
$builder
->add('body', TextareaType::class, ['attr' => ['class' => 'form-control']])
->add('receiver', EntityType::class, [
'class' => Receiver::class,
'data' => $data,
'choice_label' => 'name',
'choice_value' => "value",
'placeholder' => '- Selecteer -',
'required' => false,
'mapped' => false,
]);
}
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults([
'data_class' => WorkorderNote::class,
'mapped' => false,
]);
parent::configureOptions($resolver);
}
So I have changed mapped to false about everywhere I know. So if you could please help me with this weird problem I would be very glad.
Kind regards