Renderer::getCurrentRenderContext() triggers a TypeError when there is no current request

Problem/Motivation

We have a hook_rebuild that calls Url::fromRoute()->toString().
We calling drush cr, it works correctly. But when calling drush updb, it crashes with this error:

TypeError: SplObjectStorage::offsetExists(): Argument #1 ($object) must be of type object, null given in /var/www/web/core/lib/Drupal/Core/Render/Renderer.php on line 655 #0 /var/www/web/core/lib/Drupal/Core/Render/Renderer.php(655): SplObjectStorage->offsetExists(NULL)
#1 /var/www/web/core/lib/Drupal/Core/Render/Renderer.php(626): Drupal\Core\Render\Renderer->getCurrentRenderContext()
#2 /var/www/web/core/lib/Drupal/Core/Render/MetadataBubblingUrlGenerator.php(87): Drupal\Core\Render\Renderer->hasRenderContext()
#3 /var/www/web/core/lib/Drupal/Core/Render/MetadataBubblingUrlGenerator.php(110): Drupal\Core\Render\MetadataBubblingUrlGenerator->bubble(Object(Drupal\Core\GeneratedUrl), Array)

Steps to reproduce

Call something like this in a context where there is no current request:

// The route needs to exist.
Url::fromRoute('sitemap.page')->toString();

Proposed resolution

Renderer::getCurrentRenderContext() should check if $request is null before using it.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet