TypeError: in_array() argument #2 must be of type array, string given in preprocessAggregatorContainer()

Problem/Motivation

Certain URL's such as admin/structure/block return unexpected errors due to PHP TypeError's in function preprocessAggregatorContainer of zeropoint/src/Hook/ZeropointHooks.php.

Steps to reproduce

On Drupal 11.4.9, navigate to https://example.com/admin/structure/block.

The client shows: "The website encountered an unexpected error. Try again later."

Watchdog reports:

24366 13/Oct 23:01 php Error TypeError: in_array(): Argument #2 ($haystack) must be of type
array, string given in in_array() (line 1221 of
/var/www/drupal/themes/contrib/zeropoint/src/Hook/ZeropointHooks.p
hp).

Proposed resolution

Cast to an array when assigning classes.
$classes = (array) ($variables['element']['#attributes']['class'] ?? []);

(...or track down the guilty party who put a singleton object in there, and convince them to repent of the evil of their ways.)

Remaining tasks

User interface changes

API changes

Data model changes