title: Sonata Admin Custom field with data transformer
body: <p>I have a easy form with a data transformer, it works correctly (update, persist and delete) but I get a error on twig. </p>
  
  <p>Impossible to invoke a method ("trans") on a NULL variable ("") in SonataDoctrineORMAdminBundle:CRUD:edit_orm_one_to_many.html.twig at line 30</p>
  
  <p>error is in this line:</p>
  
  <p><code>
  {{ nested_field.vars['sonata_admin'].admin.trans(nested_field.vars.label) }}
  </code></p>
  
  <p>All fields have value in nested_field.vars['sonata_admin'] less my custom field</p>
  
  <p>My code is this:</p>
  
  <pre><code>    $formMapper
              -&gt;add(
                  $formMapper-&gt;create('articleAmount', 'text')
                      -&gt;addModelTransformer($articleAmountToStringTransformer)
              )
             ...
  </code></pre>
  
  <p>Entities </p>
  
  <pre><code>    /**
       * AppShopHasArticles
       *
       * @ORM\Table(name="app_shop_has_articles")
       * @ExclusionPolicy("all")
       * @ORM\Entity(repositoryClass="Nvia\ShopAppBundle\Entity\Repository\AppShopHasArticlesRepository")
       * @ORM\HasLifecycleCallbacks()
       */
      class AppShopHasArticles
      {
  
          /**
           * @var \Nvia\CommonBundle\Entity\Article
           *
           * @ORM\ManyToOne(targetEntity="Nvia\ShopAppBundle\Entity\Article", inversedBy="appShopHasArticles")
           * @ORM\JoinColumn(name="article_id", referencedColumnName="id", nullable=false)
           * @ORM\Id
           * @Expose
           */
          private $article;
  
          /**
           * @var \Nvia\CommonBundle\Entity\Country
           *
           * @ORM\ManyToOne(targetEntity="Nvia\CommonBundle\Entity\Country")
           * @ORM\JoinColumn(name="country_id", referencedColumnName="id", nullable=false)
           * @ORM\Id
           * @Expose
           */
          private $country;
  
          /**
           * @var \Nvia\ShopAppBundle\Entity\AppShop
           *
           * @ORM\ManyToOne(targetEntity="Nvia\ShopAppBundle\Entity\ArticleAmount")
           * @ORM\JoinColumns({
           *   @ORM\JoinColumn(name="country_id", referencedColumnName="country_id", nullable=false),
           *   @ORM\JoinColumn(name="article_id", referencedColumnName="article_id", nullable=false)
           * })
           * @Expose
           */
          private $articleAmount;
  
  
          ...
      }
  
      /**
       * ArticleAmount
       *
       * @ORM\Table(name="article_amount")
       */
      class ArticleAmount
      {
          /**
           * @var \Nvia\CommonBundle\Entity\Article
           *
           * @ORM\ManyToOne(targetEntity="Nvia\ShopAppBundle\Entity\Article", inversedBy="articleAmounts")
           * @ORM\JoinColumn(name="article_id", referencedColumnName="id", nullable=false)
           * @ORM\Id
           */
          private $article;
  
          /**
           * @var \Nvia\CommonBundle\Entity\Country
           *
           * @ORM\ManyToOne(targetEntity="Nvia\CommonBundle\Entity\Country")
           * @ORM\JoinColumn(name="country_id", referencedColumnName="id", nullable=false)
           * @ORM\Id
           */
          private $country;
  
          /**
           * @var float
           *
           * @ORM\Column(name="amount", type="float", precision=10, scale=0, nullable=false)
           * @Expose
           */
          private $amount;
  
  
          /**
           * @var \DateTime
           *
           * @ORM\Column(name="created_at", type="datetime", nullable=false)
           */
          private $createdAt;
  
      }
  </code></pre>
  
  <p>What am I doing wrong :/ ?</p>