I've cut the script down to just a few lines to try and get to the bottom of this:

import poser
fig = poser.Scene().CurrentFigure()
act = poser.Scene().CurrentActor()
mat = poser.Scene().CurrentMaterial()
mats=[]
if act and not fig:
    yn = poser.DialogSimple.YesNo(" Current Actor: "+str(act.Name())+"nnChange all materials for this actor?")
    if yn == 0:
        act = None
    else:
        mats = act.Materials()
    
poser.DialogSimple.MessageBox("Check 'if mats is None'")
if mats is None:
    poser.DialogSimple.MessageBox("mats is None")
else:
    poser.DialogSimple.MessageBox("Done 'if mats'")

I start Poser 6 from scratch with just the default ground, cameras and lights. The Main Camera is selected by default.  I run the script either from an mt5 with just a 'runPythonScript' line or direct from File > Run Python Script, makes no difference. I get the "Check 'if mats is None'" and "mats is None" messages, and then this:

poser.error: Actor has no available materials

It's definitely the highlighted mats = act.Materials() that's causing it. If I change that line to mats = None then the poser.error doesn't occur.

Seems that I'll have to avoid calling the Materials() method if the actor doesn't have any materials. Anybody have any other thoughts on this?

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).