Either the code needs to change, or we need to add some answers to a few questions in the Lesson Demo. I only looked at a MC questions in Lesson-Basic parts in QA.
Basically Lesson gets into an endless loop with a continue button for a student or a teacher in preview mode when it is on a MC question with no answer. And of course this is only possible when a existing lesson like Lesson-Basic parts exists. I have noticed for the first time a *Required on two answers in a MC question in a lesson.
In 1.9.9 and earlier, a MC question without an answer1, would display a continue button. When pressed this action would take the user to the next page in the edit order. I do not remember if it scores the question in custom or non-custom scoring mode in 1.9.9 . If the teacher skipped answer1 and recorded something in answer2, Lesson would ignore it and only put up the continue button.
In the Lesson-Basic parts demo, there are several multiple choice questions that intentionally do not have an answer. Thus the lesson does not work.
In QA I only put in an answer in the first "required" slot. It did not like it and kicked it back telling me I had missed a required field.
Willing to be part of a solution to edit the Lesson to solve this issue.