Please report such problems in the converter thread, it might get lost here in the subforums and I have notifications activated for the converter thread ;)
Concerning the problem: it's a known bug, it happens if you use the "selected objects" option and there is an object in your selection which does not have a material (like a light source or a group). This issue will be fixed in the next script version which will be released when the next public corona version is coming.
I would recommend to...
- either make sure you only select objects with materials
- better use the "convert all" option which also converts maps (The "selected" options were primarily intended for debugging conversion issues.)
It doesn't break something if you run the script several times, so you can still merge new stuff in your scene and convert again.
