Steps to reproduce
================

1. Create a new empty lesson
2. Add a single page
3. Remove the page

What happens
============

Error message:

Delete: next page not found
Stack trace:

• line 1651 of lib/deprecatedlib.php: call to debugging()
• line 27 of mod/lesson/action/delete.php: call to error()
• line 55 of mod/lesson/lesson.php: call to include()