验证失败时如何获取Symfony 4中表单字段的值？

I have two dropdowns in my form . Options in second dropdown depend on selected value in the first dropdown. If validation fails I need to get posted value of the first dropdown to set options in the second dropdown. So how to receive posted value in the form type class when validation fails?

$form = $this->createForm(MyFormType::class);

$form->handleRequest($request);

if ($form->isSubmitted() && $form->isValid()) {
    /**
     * Do your stuff when form is valid
     */
} else {
    /**
     * Here you can get the input data even failed with $form->getData()
     */
    dump($form->getData());
}

$form->getData() gives me the values of input that didn't failed in the else:

array:4 [▼
  "first_dropdown" => "hello"
]

$advert = new Advert();

    $form = $this->createForm(AdvertType::class, $advert, ['method' => 'POST'])
        ->handleRequest($request);

    if (false === $form->isSubmitted()) {
        $form->submit([]);
    }

    if (false === $form->isValid()) {
        return ['form' => $form];
    }

    $manager = $this->getDoctrine()->getManager();
    $this->appendTags($advert);
    $manager->persist($advert);
    $manager->flush();

    return $advert;

I needed something like below, in my form type class eg. DocType in buildForm() method I did:

$request = Request::createFromGlobals(); 
$myObjectId = $request->request->get('doc')['object']; // selected option of the first dropdown
$object = ($obectId) ? $objectRepository->find($objectId) : null;

Then I could use fetched object, to populate second dropdown, using query builder.