Error When Using Symfony Forms / Silex & Twig

I'm trying to render a Symfony form using twig in a Silex application. I'm using Symfony 2 Form library, Symfony Bridge Twig Forms Extension and Twig.

Here is my bootsrap file:

require_once __DIR__ . '/../app/silex.phar';
$app = new Silex\Application();

$app['debug'] = true;

// Set up the autoloader
require_once __DIR__ . '/../app/vendor/.composer/autoload.php';

// Load Configuration into container
$parser = new Symfony\Component\Yaml\Parser();
$config = $parser->parse(file_get_contents(__DIR__ . '/../app/config/config.yml'));
$app['config'] = $config;

// Set up Symfony bridge (nice twig functions)
$app->register(new Silex\Provider\SymfonyBridgesServiceProvider(), array(
'symfony_bridges.class_path' => __DIR__ . '/vendor/symfony/twig-bridge',
));

// Set Up Templating
$app->register(new Silex\Provider\TwigServiceProvider(), array(
'twig.path' => __DIR__ . '/../app/views',
'twig.class_path' => __DIR__ . '/../app/vendor/twig/twig/lib',
));

 // Set Up Database
 $app->register(new Silex\Provider\DoctrineServiceProvider(), array(
'db.options' => array(
    'driver' => $app['config']['db']['driver'],
    'host' => $app['config']['db']['host'],
    'user' => $app['config']['db']['user'],
    'password' => $app['config']['db']['password'],
    'dbname' => $app['config']['db']['dbname']
),
'db.dbal.class_path' => __DIR__ . '/vendor/doctrine/doctrine-dbal/lib',
'db.common.class_path' => __DIR__ . '/vendor/doctrine/doctrine-common/lib',
));

 // Set up the Symfony forms component
$app->register(new Silex\Provider\FormServiceProvider());

// Routes
$app->get('/', function() use($app)
{
    return $app['twig']->render('index.twig', array());
});

$app->get('/info', function() use($app)
{
    return $app['twig']->render('info.twig', array());
});

$app->get('/register', function() use($app)
{

    $builder = $app['form.factory']->createBuilder(new MppInt\Form\Type\RegisterType() )