When not throwing the error above also encountered is the Lesson not showing after selecting Show. Inconsistency of error, either the error above or not showing after selecting Show. Moving to another topic section and show/hide other activities and then coming back to the original troubling Lessons than show/hide will work.
Error also encountered for Lessons that were not indented.
Could not reproduce the error with other activities/resources, appears to be confined to the Lesson so far.
Tried switching themes but encountered the same problems (Boost/Essential).