TwigEnvironment $loader argument cannot be NULL

Problem/Motivation

The signature of TwigEnvironment::__construct() is:

  public function __construct($root, CacheBackendInterface $cache, $twig_extension_hash, StateInterface $state, LoaderInterface $loader = NULL, array $options = []) {

However, passing NULL as $loader results in a fatal error:

PHP Fatal error: Uncaught TypeError: Argument 1 passed to Twig\Environment::setLoader() must implement interface Twig\Loader\LoaderInterface, null given, called in core/lib/Drupal/Core/Template/TwigEnvironment.php on line 98 and defined in vendor/twig/twig/src/Environment.php:604

This is because

    $this->setLoader($loader);

and the setLoader() signature is

    public function setLoader(LoaderInterface $loader)

Steps to reproduce

Proposed resolution

Remove the NULL default argument.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet