\Drupal::config('system.date')->get('country.default') now has validation and can now be null

This seems like it is due to including core in the codebase which means it's probably not used directly.

Sorry, I'm not finding this piece of code used anywhere in the theme. What is this issue referring to?

Sorry my follow up comment wasn't clear, you probably won't have an issue, but you are including core in your project which is not generally recommended.

https://git.drupalcode.org/search?search=country.default&nav_source=navb...

You can see the references in the link above, feel free to close this.

I think I see the issue. You're looking at the master branch which isn't being actively used. You should be looking at: https://git.drupalcode.org/project/intercept_base/-/tree/8.x-1.x?ref_typ...

Great! This shouldn't affect you then, I'll close this!