The long answer (for me) was found in this post: Chapter 15, example code crashes
The short answer is that you can’t name the ball reference (obj_ball.scn) you put in game.scn “ball” for some reason. Maybe because it’s also named “ball” in the obj_ball.scn file? At any rate, I changed mine to “ball_ref” and things worked fine.
Also, you might run into a problem with the Accelerator part of this project like I did, the answer (for me) was here: Chapter 15 Accelerometer Issues
Seems to me like this forum format for errata is really inefficient. Wish there was a better way to organize this kind of information.