title: You can avoid this error by setting the "data_class" when use sonata_media_type
body: <p>I installed Sonata Admin and after install Sonata Media</p>
  
  <p>i have class admin for "colors" and set in configureFormFields</p>
  
  <pre><code>-&gt;add('image', 'sonata_media_type', array('required' =&gt; false,
     'context' =&gt; 'default',
     'label' =&gt; 'Image',
     'provider'=&gt;'sonata.media.provider.image'
   ))
  </code></pre>
  
  <p>While stored in the database but when edit show this error</p>
  
  <p>The form's view data is expected to be an instance of class Application\Sonata\MediaBundle\Entity\Media, but is a(n) string. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms a(n) string to an instance of Application\Sonata\MediaBundle\Entity\Media.</p>
  
  <p>Help me, please.</p>