TypeError: Illegal offset type in isset or empty in ThemeRegistry->offsetExists()

I cannot reproduce this. Here is what I did:

1. Installed Drupal 7.x on PHP 8.1 (standard install).
2. Turned on maintenance mode at /admin/config/development/maintenance.
3. Visited the home page and /node/add. There are no errors.

What else is required to reproduce the bug?

Hello, I enabled some extra error logging with

error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
$conf['error_level'] = 2;

It shows:

Deprecated: Optional parameter $data declared before required parameter $name is implicitly treated as a required parameter in /htdocs/sites/all/modules/commerce/modules/cart/commerce_cart.module on line 1202

Fatal error: Uncaught TypeError: Illegal offset type in isset or empty in /htdocs/includes/theme.inc:421 Stack trace: #0 /users/htdocs/includes/theme.inc(1150):ThemeRegistry->offsetExists() #1 /htdocs/includes/errors.inc(252): theme() #2 /htdocs/includes/bootstrap.inc(2672): _drupal_log_error() #3 [internal function]: _drupal_exception_handler() #4 {main} thrown in /htdocs/includes/theme.inc on line 421

Is this two separate unrelated errors, or Commerce → error is causing the Drupal core ThemeRegistry error?

Okay it's not related, the Commerce error can be patched with https://www.drupal.org/project/commerce/issues/3284128#comment-14616806 📌 Required parameter $options follows optional parameter $data in commerce_cart.module on line 1163 Fixed

Now it's just:

Fatal error: Uncaught TypeError: Illegal offset type in isset or empty in /htdocs/includes/theme.inc:421 Stack trace: #0 /htdocs/includes/theme.inc(1150): ThemeRegistry->offsetExists() #1 /htdocs/includes/errors.inc(252): theme() #2 /htdocs/includes/bootstrap.inc(2672): _drupal_log_error() #3 [internal function]: _drupal_exception_handler() #4 {main} thrown in /htdocs/includes/theme.inc on line 421

There are no other errors logged.