EnforceType.php

<?php

// in the controller
$user = Customer::isCustomer($this->getUser());

// in the entity
    /**
     * @param UserInterface $user
     *
     * @return Customer|null
     */
    public static function isCustomer($user)
    {
        if ($user instanceof Customer){
            return $user;
        } else {
            return null;
        }
    }