Manage Lessons: ArgumentOutOfRange Exception when deleting lessons

[davelustig]

To reproduce:

1. Start the game from the Manage Lessons screen. Verify you have at least two lessons and each lesson has at least two words.
2. Click on the second lesson and delete one of its words (so that it has at least one less word that the lesson preceding it).
3. Click on the first lesson and select its last word
4. ISSUE: Delete the lesson. This will result in everything disappearing from the screen except the background. The Unity Console will also log a series of ArgumentOutOfRange Exceptions.