title: Sonata admin "sonata_type_collection" tries to remove entity
body: <p>I have two entities and two Admin classes(StripePage and Stripe) for them. Form looks good, it has my fields and button to add new subforms. When "add new" button is clicked, new subform with form from Admin class is rendered, but if you click second time on this button or try to save entity error appears:</p>
  
  <blockquote>
    <p>Catchable Fatal Error: Argument 1 passed to
    Fred\CoreBundle\Entity\StripePage::removeStripe() must be an instance
    of Fred\CoreBundle\Entity\Stripe, null given in
    C:\Users\lubimov\OpenServer\domains\tappic.dev\src\Fred\CoreBundle\Entity\StripePage.php</p>
  </blockquote>
  
  <p>So symfony tries to remove entity instead of adding it.</p>
  
  <p>StripePageAdmin class:</p>
  
  <pre><code>class StripePageAdmin extends Admin
  {
      protected function configureListFields(ListMapper $listMapper)
      {
          $listMapper
              -&gt;addIdentifier('id', null, array('label' =&gt; 'id'))
              -&gt;add('name', null, array('label' =&gt; 'Page name'));
      }
  
      protected function configureFormFields(FormMapper $formMapper)
      {
          $formMapper
              -&gt;add('name', 'text', array('label' =&gt; 'Page name'))
              -&gt;add('stripes', 'sonata_type_collection',
                  array('label' =&gt; 'Stripes', 'required' =&gt; false, 'by_reference' =&gt; false),
                  array('edit' =&gt; 'inline', 'inline' =&gt; 'table', 'sortable' =&gt; 'pos')
              )
              -&gt;end();
      }
  }
  </code></pre>
  
  <p>StripeAdmin class:</p>
  
  <pre><code>class StripeAdmin extends Admin
  {
      protected function configureListFields(ListMapper $listMapper)
      {
          $listMapper
              -&gt;addIdentifier('id')
              -&gt;add('picture', null, array('sortable' =&gt; true))
              -&gt;add('text', null, array('sortable' =&gt; true))
              -&gt;add('deep_link', null, array('sortable' =&gt; true));
      }
  
      protected function configureFormFields(FormMapper $formMapper)
      {
          $formMapper
              -&gt;add('file', 'file', array('label' =&gt; 'Picture'))
              -&gt;add('text', null, array('label' =&gt; 'Text'))
              -&gt;add('deep_link', 'choice', array(
                  'choices'  =&gt; array('test1' =&gt; 'Test1', 'test' =&gt; 'Test2'),
                  'label' =&gt; 'Deep Link',
              ))
              -&gt;end();
      }
  }
  </code></pre>
  
  <p>What is the problem? Stripe form in admin class must be configured by other way?</p>