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

Problem/Motivation

Got the error while testing against the bootstrap theme, 8.x-3.25

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