title: Use Sonata Field Type on normal Form Class
body: <p>I'm trying to insert custom sonata form field type on the front page, not in SonataAdmin, something like this:</p>
  
  <pre><code> $form = $this-&gt;createFormBuilder($content)
              -&gt;add('titleEs', 'text', array('required' =&gt; true, 'label' =&gt; 'label.title.spanish', 'attr' =&gt; array('class' =&gt; 'col-xs-12 form-control input-lg')))
              -&gt;add('contentEs', 'ckeditor', array('required' =&gt; true,'label' =&gt; 'label.content.spanish', 'attr' =&gt; array('class' =&gt; 'col-xs-12')))
              -&gt;add('titleEn', 'text', array('required' =&gt; true,'label' =&gt; 'label.title.english', 'attr' =&gt; array('class' =&gt; 'col-xs-12 form-control input-lg')))
              -&gt;add('contentEn', 'ckeditor', array('required' =&gt; true, 'label' =&gt; 'label.content.english', 'attr' =&gt; array('class' =&gt; 'col-xs-12')))
              -&gt;add('header', 'sonata_type_model', array('required' =&gt; true,'label' =&gt; 'label.content.headerImage'), array('link_parameters' =&gt; array('context' =&gt; 'content/front', 'size' =&gt; 'big')))
              //-&gt;add('coverImage', 'sonata_type_model_list', array('required' =&gt; true,'label' =&gt; 'label.content.coverImage'), array('link_parameters' =&gt; array('context' =&gt; 'content/front', 'size' =&gt; 'small')))
              //-&gt;add('sliderImage', 'sonata_type_model_list', array('required' =&gt; false,'label' =&gt; 'label.content.sliderImage'), array('link_parameters' =&gt; array('context' =&gt; 'content/slider', 'size' =&gt; 'normal')))
              -&gt;getForm(); 
  </code></pre>
  
  <p>But when I execute that, it throws an error:</p>
  
  <pre><code>Catchable Fatal Error: Argument 1 passed to Sonata\AdminBundle\Form\ChoiceList\ModelChoiceList::__construct() must implement interface Sonata\AdminBundle\Model\ModelManagerInterface, null given
  </code></pre>
  
  <p>I can't understand why Symfony throws that error, if the Sonata Form Field Types are services.</p>