Fix deprecation notice with symfony/twig-bridge 3.2+ in TwigServiceProvider

src/Silex/Provider/TwigServiceProvider.php

@@ -109,7 +109,7 @@ class TwigServiceProvider implements ServiceProviderInterface
                         return new TwigRenderer($app['twig.form.engine'], $csrfTokenManager);
                     };
 
-                    $twig->addExtension(new FormExtension($app['twig.form.renderer']));
+                    $twig->addExtension(new FormExtension(class_exists(HttpKernelRuntime::class) ? null : $app['twig.form.renderer']));
 
                     // add loader for Symfony built-in form templates
                     $reflected = new \ReflectionClass('Symfony\Bridge\Twig\Extension\FormExtension');