Expected argument of type "App\Entity\Theme", "Doctrine\Common\Collections\ArrayCollection" given at property path "themes"

I have a many to many relationship between Customer, Formation and Theme entities. The insertion form of Customer does not work.

I explain what I want to do : I want to add "Customer" by adding one or more "Formation" and one or more "Themes" knowing that the theme are related to "Formation", ie if a customer has the Training_1, he will not be able to have Theme from Formation_2.

All the code is there.

Customer Entity :

<?php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;
use Symfony\Component\Form\Form;

/**
 * @ORM\Entity(repositoryClass="App\Repository\FormationRepository")
 * @ORM\Table(name="lol_customer")
 */
class Customer
{
    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=180, unique=true)
     */
    private $customerName;

    /**
     * @var Formation[]|ArrayCollection
     *
     * @ORM\ManyToMany(targetEntity="App\Entity\Formation", cascade={"persist"})
     * @ORM\JoinTable(name="lol_customer_formation")
     * @ORM\OrderBy({"id": "ASC"})
     */
    private $formations;

    /**
     * @var Theme[]|ArrayCollection
     *
     * @ORM\ManyToMany(targetEntity="App\Entity\Theme", cascade={"persist"})
     * @ORM\JoinTable(name="lol_customer_theme")
     * @ORM\OrderBy({"id": "ASC"})
     */
    private $themes;

    /**
     * @ORM\Column(type="boolean")
     */
    private $enabled;

    /**
     * @var \DateTime
     *
     * @ORM\Column(type="datetime")
     */

    private $startDate;
    /**
     * @var \DateTime
     *
     * @ORM\Column(type="datetime")
     */
    private $endDate;

    public function __construct()
    {
        $this->startDate = new \DateTime();
        $this->endDate = new \DateTime();
        $this->formations = new ArrayCollection();
    }



    public function getId(): ?int
    {
        return $this->id;
    }

    public function setFormations(Formation ...$formations): void
    {
        foreach ($formations as $formation) {
            if (!$this->formations->contains($formation)) {
                $this->formations->add($formation);
            }
        }
    }

    public function getFormations()
    {
        return $this->formations;
    }

    public function setThemes(Theme ...$themes): void
    {
        foreach ($themes as $theme) {
            if (!$this->themes->contains($theme)) {
                $this->themes->add($theme);
            }
        }
    }

    public function getThemes()
    {
        return $this->themes;
    }

    /**
     * @return CustomerName
     */
    public function getCustomerName()
    {
        return $this->customerName;
    }

    public function setCustomerName($customerName): self
    {
        $this->customerName = $customerName;

        return $this;
    }

    /**
     * @return bool
     */
    public function getEnabled(): bool
    {
        return $this->enabled;
    }

    /**
     * @param bool
     * @return self
     */
    public function setEnabled($enabled): self
    {
        $this->enabled = $enabled;

        return $this;
    }

    public function getStartDate(): \DateTime
    {
        return $this->startDate;
    }

    public function setStartDate(\DateTime $startDate): void
    {
        $this->startDate = $startDate;
    }

    public function getEndDate(): \DateTime
    {
        return $this->endDate;
    }

    public function setEndDate(\DateTime $endDate): void
    {
        $this->endDate = $endDate;
    }

    public function __toString()
    {
        return $this->customerName;
    }

}

Formation Entity :

<?php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * @ORM\Entity()
 * @ORM\Table(name="lol_formation")
 *
 */
class Formation implements \JsonSerializable
{
    /**
     * @var int
     *
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(type="string", unique=true)
     */
    private $name;

    /**
     * @var Theme[]|ArrayCollection
     *
     * @ORM\ManyToMany(targetEntity="App\Entity\Theme", cascade={"persist"})
     * @ORM\JoinTable(name="lol_formation_theme")
     * @ORM\OrderBy({"name": "ASC"})
     */
    private $themes;

    public function __construct()
    {
        $this->themes = new ArrayCollection();
    }

    public function getId(): ?int
    {
        return $this->id;
    }

    public function setName(string $name): void
    {
        $this->name = $name;
    }

    public function getName(): ?string
    {
        return $this->name;
    }

    /**
     * {@inheritdoc}
     */
    public function jsonSerialize(): string
    {
        return $this->name;
    }

    public function getThemes(): ?array
    {
        return $this->themes;
    }

    public function setThemes(?array $themes): self
    {
        $this->themes = $themes;

        return $this;
    }

    public function __toString(): string
    {
        return $this->name;
    }
}

Theme Entity :

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity()
 * @ORM\Table(name="lol_Theme")
 *
 */
class Theme implements \JsonSerializable
{
    /**
     * @var int
     *
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(type="string", unique=true)
     */
    private $name;

    public function getId(): ?int
    {
        return $this->id;
    }

    public function setName(string $name): void
    {
        $this->name = $name;
    }

    public function getName(): ?string
    {
        return $this->name;
    }

    /**
     * {@inheritdoc}
     */
    public function jsonSerialize(): string
    {    
        return $this->name;
    }

    public function __toString(): string
    {
        return $this->name;
    }
}

CustomerType (form of Customer) :

<?php

namespace App\Form;

use App\Entity\Customer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolver;
use App\Form\Type\DateTimePickerType;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;

class CustomerType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('customerName', null, array(
                    'label'    => 'Domaine du client ',
                    'required' => true)
            )
            ->add('enabled', null, array(
                'label'    => 'Activer la formation'), [
                'required'   => false,
            ])
            ->add('formations', EntityType::class, [
                'class' => "App\Entity\Formation",
                'choice_label' => 'name',
                'multiple' => true,
                'expanded' => true,
            ])
            ->add('themes', EntityType::class, [
                'class' => "App\Entity\Theme",
                'choice_label' => 'name',
                'multiple' => true,
                'expanded' => true,
            ])
            ->add('startDate', DateTimePickerType::class, [
                'label' => 'Date de début de la formation',
            ])
            ->add('endDate', DateTimePickerType::class, [
                'label' => 'Date de fin de la formation',
            ]);
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Customer::class,
            'customerName' => null
        ]);
    }
}

I have the following error :

Expected argument of type "App\Entity\Theme", "Doctrine\Common\Collections\ArrayCollection" given at property path "themes".

Do you know where this error comes from? Thank you

Inside constructor of customer entity you are missing initialization for "themes" array collection.