Thanks for responding. The problem was happening when I pressed the Play RGBullsEye button. However, it is no longer happening. I disconnected everything and reworked steps 3 and 4. Now it works. Yesterday, I scrutinized the code carefully, but perhaps I made some erroneous connection; or, maybe was just a fluke. Who knows? Either way it is working now.
I appreciate how much work you and your team put into these books, and it feels comforting to know that the authors take pride in their work and want to make a great product. Thanks!