title: Calling $builder->getData() from within a nested form always returns NULL
body: <p>I'm trying to get data stored in a nested form but when calling <code>$builder-&gt;getData()</code> I'm always getting NULL.</p>
  
  <p>Does anyone knows what how one should get the data inside a nested form?</p>
  
  <p>Here's the ParentFormType.php:</p>
  
  <pre><code>class ParentFormType extends AbstractType
  {
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder-&gt;add('files', 'collection', array(
              'type'          =&gt; new FileType(),
              'allow_add'     =&gt; true,
              'allow_delete'  =&gt; true,
              'prototype'     =&gt; true,
              'by_reference'  =&gt; false
          );
      }
  }
  </code></pre>
  
  <p>FileType.php</p>
  
  <pre><code>class FileType extends AbstractType
  {
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          // Each one of bellow calls returns NULL
          print_r($builder-&gt;getData());
          print_r($builder-&gt;getForm()-&gt;getData());
          die();
  
          $builder-&gt;add('file', 'file', array(
              'required'    =&gt; false,
              'file_path'   =&gt; 'file',
              'label'       =&gt; 'Select a file to be uploaded',
              'constraints' =&gt; array(
                  new File(array(
                      'maxSize' =&gt; '1024k',        
                  ))
              ))
          );
      }
  
      public function setDefaultOptions( \Symfony\Component\OptionsResolver\OptionsResolverInterface $resolver )
      {
          return $resolver-&gt;setDefaults( array() );
      }
  
      public function getName()
      {
          return 'FileType';
      }
  }
  </code></pre>
  
  <p>Thanks!</p>