How to use the User value resolver in Symfony 3.2? - symfony

Question

I'm building a small website using symfony 3.2.
There is a page in which the user can change its profile data with a form. I used the structure seen in the official tutorial. Here is the declaration of my controller :
/** #Route("/profil", name="profil_show_user") */
public function userProfileAction(Request $request, UserInterface $user) {
if (!$this->get('security.authorization_checker')->isGranted('IS_AUTHENTICATED_FULLY')) {
throw $this->createAccessDeniedException();
}
//...
$user->getProfile();//and do stuff
//...
}
My problem is that if the user is disconnected for being inactive for too long, or if someone bookmark the page but is not connected, I have this ugly error coming :
Controller
"AppBundle\Controller\ProfilController::userProfileAction()" requires
that you provide a value for the "$user" argument. Either the argument
is nullable and no null value has been provided, no default value has
been provided or because there is a non optional argument after this
one. 500 Internal Server Error - RuntimeException
In the New in Symfony 3.2 changelog, there is something about the new User value resolver. I tried to change to UserInterface $user = null, and it make the page redirect to the path I set in failure_path of security.yml, which is the good behaviour.
But then if I'm connected and go to profil_show_user, I get that other error :
Error: Call to a member function getProfile() on null
I search thoroughly the symfony documentation but couldn't find anything.
Could someone explain to me what goes wrong, what I misunderstood and how can I make this work ?
EDIT :
I thought I might say that if I don't use te value resolver, everything works fine. This is an educationnal and curiosity question about a new feature which I don't manage to use. This code works :
/** #Route("/profil", name="profil_show_user") */
public function userProfileAction(Request $request) {
$user = $this->getUser();
if (!$this->get('security.authorization_checker')->isGranted('IS_AUTHENTICATED_FULLY')) {
throw $this->createAccessDeniedException();
}
//...
$user->getProfile();//and do stuff
//...
}