Undefined constant "DRUPAL_TEST_IN_CHILD_SITE"

Problem/Motivation

I get a 500 error when accessing /core/install.php.

Steps to reproduce

Build the Drupal site, activate error_page and access the URL "/core/install.php".

The output to the error log is as follows:
2024/04/02 17:38:06 [error] 7548#7548: *27940 FastCGI sent in stderr: "PHP message: PHP Fatal error: During class fetch: Uncaught Error: Undefined constant "Drupal\error_page\DRUPAL_TEST_IN_CHILD_SITE" in /var/www/drupal/web/modules/contrib/error_page/src/ErrorPageErrorHandler.php:68
Stack trace:
#0 [internal function]: Drupal\error_page\ErrorPageErrorHandler::handleError(16384, 'Since symfony/d...', '/var/www/drupal...', 25)
#1 /var/www/drupal/vendor/symfony/deprecation-contracts/function.php(25): trigger_error('Since symfony/d...', 16384)
#2 /var/www/drupal/vendor/symfony/dependency-injection/ContainerAwareTrait.php(14): trigger_deprecation('symfony/depende...', '6.4', '"%s" is depreca...', 'Symfony\\Compone...')
#3 /var/www/drupal/vendor/composer/ClassLoader.php(576): include('/var/www/drupal...')
#4 /var/www/drupal/vendor/composer/ClassLoader.php(427): Composer\Autoload\{closure}('/var/www/drupal...')
#5 /var/www/drupal/web/core/lib/Drupal/Core/Logger/LoggerChannelFactory.php(12): Comp" while reading response header from upstream, client: 10.1.51.40, server: localhost, request: "GET /core/install.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php-fpm/www.sock:", host: "localhost"

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes