Symfony\Component\DependencyInjection\ContainerInterface::get() can return null when using ContainerInterface::NULL_ON_INVALID_REFERENCE

$stack = $container->get('request_stack', ContainerInterface::NULL_ON_INVALID_REFERENCE);
if (null === $stack) {
 return null;
}

Strict comparison using === between null and Symfony\Component\HttpFoundation\RequestStack will always evaluate to false.