I can type
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You're testing all of the objects that were selected when the script reached the line
fbxs = [ ob for ob in bpy.context.selected_objects]
One of these objects has no parent, so when the line
ob.parent.select = True is run, Blender tries to find a parent and raises an error when it doesn't find one.
Try this instead:
if (ob.parent): ob.parent.select
or if that doesn't work for some reason, enclose it in a try:/except: block.