Using the fast404.inc approach throws errors

Loading the fast404.inc file from settings.php and calling fast404_preboot($settings); is broken. This approach is outlined in the README.txt.

When $settings['fast404_path_check'] = TRUE;, Drupal attempts to run db queries and access the service container without either of these being established.

The website encountered an unexpected error. Please try again later.
Drupal\Core\DependencyInjection\ContainerNotInitializedException: \Drupal::$container is not initialized yet. \Drupal::setContainer() must be called with a real container. in Drupal::getContainer() (line 129 of /var/www/docroot/core/lib/Drupal.php).

When $fast_404->extensionCheck(); is run, Drupal will return Cannot modify header information errors.

RuntimeException: Failed to start the session because headers have already been sent by "/var/www/sitename/vendor/symfony/http-foundation/Response.php" at line 376. in Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() (line 141 of /var/www/sitename/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php) #0
/var/www/sitename/docroot/core/lib/Drupal/Core/Session/SessionManager.php(164): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() #1 /var/www/sitename/docroot/core/lib/Drupal/Core/Session/SessionManager.php(195): Drupal\Core\Session\SessionManager->startNow() #2 /var/www/sitename/vendor/symfony/http-foundation/Session/Session.php(196): Drupal\Core\Session\SessionManager->save() #3 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(60): Symfony\Component\HttpFoundation\Session\Session->save() #4 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #5 /var/www/sitename/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(184): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #6 /var/www/sitename/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(121): Drupal\page_cache\StackMiddleware\PageCache->fetch(Object(Symfony\Component\HttpFoundation\Request), 1, true) #7 /var/www/sitename/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(75): Drupal\page_cache\StackMiddleware\PageCache->lookup(Object(Symfony\Component\HttpFoundation\Request), 1, true) #8 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #9 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #10 /var/www/sitename/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #11 /var/www/sitename/docroot/core/lib/Drupal/Core/DrupalKernel.php(666): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #12 /var/www/sitename/docroot/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #13 {main}.

Warning: Cannot modify header information - headers already sent by (output started at /var/www/sitename/vendor/symfony/http-foundation/Response.php:376) in Drupal\fast404\Fast404->response() (line 202 of /var/www/sitename/docroot/modules/contrib/fast_404/src/Fast404.php) #0
 /var/www/sitename/docroot/core/includes/bootstrap.inc(582): _drupal_error_handler_real(2, 'Cannot modify h...', '/var/www/sitename/do...', 202, Array) #1 [internal function]: _drupal_error_handler(2, 'Cannot modify h...', '/var/www/sitename/do...', 202, Array) #2 /var/www/sitename/docroot/modules/contrib/fast_404/src/Fast404.php(202): header('HTTP/1.0 404 No...') #3 /var/www/sitename/docroot/modules/contrib/fast_404/src/EventSubscriber/Fast404EventSubscriber.php(52): Drupal\fast404\Fast404->response(true) #4 [internal function]: Drupal\fast404\EventSubscriber\Fast404EventSubscriber->onKernelRequest(Object(Symfony\Component\HttpKernel\Event\GetResponseEvent), 'kernel.request', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher)) #5 /var/www/sitename/docroot/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(111): call_user_func(Array, Object(Symfony\Component\HttpKernel\Event\GetResponseEvent), 'kernel.request', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher)) #6 /var/www/sitename/vendor/symfony/http-kernel/HttpKernel.php(127): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.request', Object(Symfony\Component\HttpKernel\Event\GetResponseEvent)) #7 /var/www/sitename/vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1) #8 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #9 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #10 /var/www/sitename/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(184): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #11 /var/www/sitename/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(121): Drupal\page_cache\StackMiddleware\PageCache->fetch(Object(Symfony\Component\HttpFoundation\Request), 1, true) #12 /var/www/sitename/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(75): Drupal\page_cache\StackMiddleware\PageCache->lookup(Object(Symfony\Component\HttpFoundation\Request), 1, true) #13 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #14 /var/www/sitename/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #15 /var/www/sitename/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #16 /var/www/sitename/docroot/core/lib/Drupal/Core/DrupalKernel.php(666): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #17 /var/www/sitename/docroot/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #18 {main}.