about an exercise in "Vizard Teacher in a book"

[Jeff]

Are you trying to modify the example code included in the tutorial? The original code adds a window model that has a glass sub-part:

#Add a model and grab one of its chidren to put the textures on.
model = viz.add('art/window.ive')
window = model.getChild( 'glass' )

If you open the 'window' model in Inspector you'll see the scene graph and model sub-parts. In your code 'window' was swapped with 'wall'. The 'wall' model does not have a sub-part called 'glass'.