You can perform delete action on non-empty content type group, and you'll get error, that it's not empty. Probably it should be resolved as in Sections.

Prerequisites:

1. Content Type Group with at least one Content Type in

Steps:

1. Go to Admin > Content Types
2. Check checkbox left to content type group from prerequisites
3. Click Delete button

Actual result:

Argument '$contentTypeGroup' is invalid: Content type group has content type instances

Expected result:

Checkboxes are disabled for groups containing content types.