In Moodle 4.5, reactive JS searches for the course add sections button, and changes it to a message if the maximum number of sections has been reached.  However it tries to do this even on single section pages, where there is no such button, and it gives an error.

Steps to reproduce:

1. Create a new Custom Sections format course.
2. Turn editing on.
3. Open the JavaScript console, to show error messages.
4. Click on a section in the course index, to go to the section's page.
5. An error message "TypeError: courseAddSections is null" appears in the console.