Saving the inverse side of a many-to-many relation

Question

I try to make it possible to save a relation on the inversed side of the relation. I know that this isn't possible in Doctrine but I found various articles which show a possibility. The problem is that the described way doesn't work for me.

My starting basis:

I have two entities within a many-to-many relation.

// Widget.php

/**
* @ORM\ManyToMany(targetEntity="App\Entity\Layout", mappedBy="widgets", cascade={"persist"})
*/
private $layouts;

// Layout.php

/**
* @ORM\ManyToMany(targetEntity="App\Entity\Widget", inversedBy="layouts", cascade={"persist"})
*/
private $widgets;

I have for both entities the getter, setter and an add and remove method. They looks like this (both the same way):

public function setLayouts(ArrayCollection $layouts) {
    $this->layouts->clear();
    foreach ($layouts as $layout) {
        $this->addLayout($layout);
    }
    return $this;
}

public function getLayouts() {
    return $this->layouts;
}

public function addLayout(Layout $layout) {
    if ($this->layouts->contains($layout)) {
        return $this;
    }
    $this->layouts->add($layout);
    $layout->addWidget($this);
    return $this;
}

public function removeLayout(Layout $layout) {
    if (!$this->layouts->contains($layout)) {
        return $this;
    }
    $this->layouts->removeElement($layout);
    $layout->removeWidget($this);
    return $this;
}

So my owning side is layout in this case. Additionally I have the FormTypes for both entities:

class LayoutType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder->add('widgets', EntityType::class, [
            'required' => true,
            'class' => Widget::class,
            'multiple' => true,
        ]);
...

class WidgetType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder->add('layouts', EntityType::class, [
            'required' => false,
            'class' => Layout::class,
            'multiple' => true,
            'by_reference' => false,
        ]);
...

And finally my controller (The layout controller looks the same):

public function postAction(Request $request) {
    $widget = new Widget();

    $formFactory = $this->container->get('form.factory');
    $form = $formFactory->createNamed('', WidgetType::class, $widget, ['method' => 'POST']);
    $form->handleRequest($request);

    if (!$form->isValid()) {
        return View::create($form, Response::HTTP_BAD_REQUEST);
    }

    $em = $this->getDoctrine()->getManager();
    $em->persist($widget);
    $em->flush();

    return View::create($widget);
...

The possible solution:

I'm using a Symfony form for all my entities, so it should be possible to add a 'by_reference' => false to the specific field. I found this solution for example here: https://afilina.com/doctrine-not-saving-manytomany As you see above I added the option already.

My problem:

The first side, saving a Layout and add widgets to it, works perfectly. But the other side, saving a Widget and add layouts to it doesn't work. Why is that so? I'm using the latest version of Smyfony (4.0.10). Is that maybe the problem?

Thanks for help!