Hmm, weird. If I read the script right, it'll test each actor for being a prop - if not, go to the next actor.
It should only reach the try: statement when the actor is a prop, and it should only reach the for statement if the Materials() function returns a value.

What might work is the following:

for actor in scene.Actors():
    if not actor.IsProp():
        continue
    try:
        materials=actor.Materials()
    except:
        continue
    if !materials:
        continue
    for mat in materials:
        ....

The red code should prevent the error from happening.

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