array_diff(): Argument #2 must be of type array, null given in array_diff() (line 702 [...] core/lib/Drupal/Core/Theme/Registry.php)

Also got this error. It happened for me when I ran database updates on my live server without access to Drush. Updates worked fine locally with Drush.

Same issue here with core version 10.3.1

 [warning] Undefined array key "preprocess functions" Registry.php:702

In Template.php line 385:

  An exception has been thrown during the rendering of a template ("array_diff(): Argument #2 must be of type array, null given") in "__string_template__..." at line 1.

In Registry.php line 702:

  array_diff(): Argument #2 must be of type array, null given

According to my previous comment, updating to 10.3.2 solved this issue for me.

Can be related to theme that used on exact site.
F.e. see: https://www.drupal.org/project/bootstrap/issues/3460442 🐛 Cannot run update.php Active

Sounds like a duplicate of 🐛 Exception: Warning: Undefined array key "preprocess functions" Drupal\Core\Theme\Registry->mergePreprocessFunctions() Needs work