TypeError: in_array(): Argument #2 ($haystack) must be of type array, null given

Please review.

I am not able to reproduce it. Even if all Readmehelp modules are disabled on the /admin/config/system/readmehelp-settings page we get an empty array here:

$readmehelp_modules = \Drupal::config('readmehelp.settings')
  ->get('readmehelp_modules');

As for the second error you do not need to depend on the Readmehelp in your custom modules anymore. It scans all enabled modules and if some of them has no hook_help() implementation and has README.md in its directory then it will be rendered on a /admin/help/your_module page.

You can manually disable / enable the Readmehelp support on the settings page above. If you are unsure which modules are eligible for this kind of support just reinstall the Readmehelp and it will discover that for you automatically.

See more: 📌 Allow using the readmehelp for all modules with no module help. Fixed

Reviewed this issue and the error not present for me too after installing and taking the module page.
D10.1.1, php 8.2