Blank page after submitting form

require __DIR__ . '/vendor/autoload.php';
date_default_timezone_set('Europe/Belfast');

//$log = new Monolog\Logger('name');
//$log->pushHandler(new Monolog\Handler\StreamHandler('app.txt', Monolog\Logger::WARNING));
//$log->addWarning('Foo');



$app = new \Slim\Slim( array(
'view' => new \Slim\Views\Twig()
));


$view = $app->view();
$view->parserOptions = array(
    'debug' => true,

);

$view->parserExtensions = array(
    new \Slim\Views\TwigExtension(),
);

$app->get('/', function() use($app){
  $app->render('about.twig');
})->name('home');

$app->get('/contact', function() use($app){
  $app->render('contact.twig');
})->name('contact');

$app->post('/contact', function() use($app){
  $name = $app->request->post('name');
  $email = $app->request->post('email');
  $msg = $app->request->post('msg');

  if(!empty($name) && !empty($email) && !empty($msg)){
    $cleanName = filter_var($name, FILTER_SANITIZE_STRING); 
    $cleanEmail = filter_var($email, FILTER_SANITIZE_EMAIL);
    $cleanMsg = filter_var($msg, FILTER_SANITIZE_STRING);

  } else {
   $app->redirect('/contact'); 
  }

  $transport = Swift_SendMailTransport::newInstance('/usr/sbin/sendmail -bs');
  $mailer = \Swift_Mailer::newInstance($transport);

  $message = \Swift_Mailer::newInstance();
  $message->setSubject('Email from our website');
  $message->setFrom(array($cleanEmail => $cleanName));

  $message->setTo(array('treehouse@localhost'));
  $message->setBody($cleanMsg);

  $result = $mailer->send($message);

  if($result > 0){
  //send a message to thank
  $app->redirect('/');  
  } else {
  //message failed
  //log error
  $app->redirect('/contact');
  }


});

$app->run();



?>

Code for form:
    <form action="" method="post"> 
      <fieldset>
        <input name="name" type="text" placeholder="Full Name">
        <input name="email" type="email" placeholder="Email Address">
        <textarea name="msg" placeholder="Your message..."></textarea>
      </fieldset>
      <input type="submit" class="button">
    </form>```