Cannot use form extensions about bootstrap-bundle

[althaus]

You have to use the FQDN since Symfony 3:

use Braincrafted\Bundle\BootstrapBundle\Form\Type\FormActionsType;

class FooType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('actions', FormActionsType::class);
    }
}