Yes, the GeometryObject is outdated - since the newer version of the Farseer engine some things changed. I updated those docs on the home page to reflect the difference.
So for your current crash, I am guessing that the sprite "ball" does not exist.
Place a breakpoint on that line where you access PhysicsObjects, then go to the Watch Window and check the _physicsController.PhysicsObjects dictionary (you can look at the keys like so):
... just change the (0) to 1,2,3, etc. Or better yet add a debug method that writes them out to the debug window.
This will show you all of your sprite key values that are valid to access.