Symfony3 form checkbox save error

I tried to look up on Google but didn't find anyone with such a problem. I think I did everything like the documentation guides but I guess I'm missing something

So I have a form with checkbox like this:

    $builder->add(
        'productTypes',
        EntityType::class,
        array(
            'label'        => 'Available for products',
            'class'        => 'ShopBundle:ProductType',
            'choice_label' => 'name',
            'multiple'     => true,
            'expanded'     => true,
            'by_reference' => false,
        )
    );

When I'm editing everything goes smooth, I can edit existing entry and check or uncheck this checkbox, it saves properly, but when I try to add new Object I get error:

PHP Fatal error: Call to a member function add() on null in C:\xampp\htdocs\uniacar-sf\src\ShopBundle\Entity\ProductAttribute.php on line 188

This is my controller action:

public function editAction(Request $request, $id = null)
{
    $this->setMenuTab('cars', 'admin');
    $productTypes = new ArrayCollection();

    if (!empty($id)) {
        $attribute = $this->getRepo(ProductAttribute::class)->find($id);
        $this->setTitle('admin.cars.attributes.edit');

        foreach ($attribute->getProductTypes() as $value) {
            $productTypes->add($value);
        }
    } else {
        $attribute = new ProductAttribute();
        $this->setTitle('admin.cars.attributes.new');
    }

    $form = $this->createForm(ProductAttributeForm::class, $attribute);
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        $attribute = $form->getData();

        foreach ($productTypes as $value) {
            if (false === $attribute->getProductTypes()->contains($value)) {
                $attribute->getProductTypes()->removeElement($value);
                $this->db()->remove($value);
            }
        }

        $this->db()->persist($attribute);
        $this->db()->flush();

        return $this->redirectToRoute('carAdmin', array('tab' => 'attributes'));
    }

    $this->setVariables(
        array(
            'form'      => $form->createView(),
            'attribute' => $attribute,
        )
    );

    return $this->response();
}

$this->db() is my shortcut for $this->getDoctrine()->getManager()

And this is definition part of ProductAttribute that relates to ProductType:

/**
 * Constructor
 */
public function __construct() {
    $this->productTypes = new ArrayCollection();
}

/**
 * Many Attributes have Many ProductTypes
 * @ORM\ManyToMany(targetEntity="ProductType", mappedBy="attributes", cascade={"persist"})
 */
private $productTypes;

/**
 * @param ProductType $productType
 */
public function addProductType(ProductType $productType)
{
    $this->productTypes->add($productType);
    $productType->addProductAttribute($this);
}

/**
 * @param ProductType $productType
 */
public function removeProductType(ProductType $productType)
{
    $this->productTypes->removeElement($productType);
}

Also there is part of ProductType Entity that relates to ProductAttribute:

/**
 * Constructor
 */
public function __construct() {
    $this->attributes = new ArrayCollection();
}

/**
 * Many ProductTypes have Many Attributes
 * @ORM\ManyToMany(targetEntity="ProductAttribute", inversedBy="productTypes")
 * @ORM\JoinTable(name="product_type_to_attribute")
 */
private $attributes;


/**
 * @param ProductAttribute $attribute
 */
public function addProductAttribute(ProductAttribute $attribute)
{
    if (!$this->attributes->contains($attribute)) {
        $this->attributes->add($attribute);
    }
}

public function removeProductAttribute(ProductAttribute $attribute)
{
    $this->attributes->removeElement($attribute);
}

I tried to follow Symfony Embed Form Tutorial (How to Embed a Collection of Forms) I know that in this case there is no embeded collection (I have another field in this Entity, that is embeded collection of forms and it works just fine) but from what I understand relations are the same in this case, it's many to many so I have to tell the Symfony how to treat relations, add and remove objects. I dumped data that comes in POST but it's the same as for edition - productType is there. Any ideas why do I get this error?

It fires in ProductAttribute Entity in the line $this->productTypes->add($productType);