User is able to remove all lessons

[humitos]

Steps to reproduce it:

1. Open Typing Turtle
2. Go to Edit Lessons screen
3. Remove all lessons with the "minus" sign (it's not possible to remove the the last one)
4. Click on "Save lessons to Activity"
5. Close Typing Turtle
6. Start a New Instance (by holding Alt)

Issue:

Just one lesson is available to play

Expected issue:

I think it shouldn't be possible to save the lessons to the Activity . This is dangerous because we can break the activity by mistake.