Cause i got an exception, when leaving that line in the form type:
```
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class'   => 'Symfony\Cmf\Bundle\SeoBundle\Model\ExtraProperty',
...
``

@WouterJ i think to extend the collection form itself right?