title: Symfony / FOSUserBundle : overriding error
body: <p>I'm running Symfony 3.4 and FOSUserBundle (master version).
  I added 2 new fields in my custom User entity :</p>
  
  <ul>
  <li>firstname</li>
  <li>lastname</li>
  </ul>
  
  <p>.. and I'm now trying to edit the profile edit form with those 2 new fields.
  I followed this tutorial : <a href="https://symfony.com/doc/master/bundles/FOSUserBundle/overriding_forms.html" rel="nofollow noreferrer">Overriding Default FOSUserBundle Forms</a>, this is my custom Form :</p>
  
  <pre><code>&lt;?php
  
  namespace MyCompany\PimCoreBundle\Form\Type;
  
  use FOS\UserBundle\Util\LegacyFormHelper;
  use Symfony\Component\Form\AbstractType;
  use Symfony\Component\Form\FormBuilderInterface;
  use Symfony\Component\OptionsResolver\OptionsResolver;
  use Symfony\Component\Security\Core\Validator\Constraints\UserPassword;
  use Symfony\Component\Validator\Constraints\NotBlank;
  
  class ProfileFormType extends AbstractType
  {
  
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          parent::buildForm($builder, $options);
          $builder-&gt;add('firstname', 'text', array('label' =&gt; 'Firstname'));
          $builder-&gt;add('lastname', 'text', array('label' =&gt; 'Lastname'));
      }
  
      public function getParent()
      {
          return 'FOS\UserBundle\Form\Type\ProfileFormType';
      }
  
      public function getBlockPrefix()
      {
          return 'app_user_profile';
      }
  
  }
  
  
  ?&gt;
  </code></pre>
  
  <p>my config :</p>
  
  <pre><code>fos_user:
      db_driver: orm # other valid values are 'mongodb' and 'couchdb'
      firewall_name: main
      user_class: MyCompany\PimCoreBundle\Entity\User
      from_email:
          address: "contact@MyCompany.com"
          sender_name: "MyCompany"
      profile:
          form:
              type: MyCompany\PimCoreBundle\Form\Type\ProfileFormType
  </code></pre>
  
  <p>.. and my services :</p>
  
  <pre><code>services:
      app.profile.form.type:
          class: MyCompany\PimCoreBundle\Form\Type\ProfileFormType
          tags:
              - { name: form.type, alias: app_user_profile }
  </code></pre>
  
  <p>I made the changes in the config + services ymal files.
  But I get this error :</p>
  
  <p><strong>Could not load type "app_user_profile": class does not exist.</strong></p>
  
  <p>Can't understand what's wrong and how tu debug it !</p>