I have a PRTVolume that has a material applied. The material uses a VertexColor map to access mapping channel 5 to modify the density in a KrakatoaMtl.
If you set the VertexMap to a mapping channel that does not exist you get an empty error box when you try to render. It just has a red X and an OK button. Would be helpful if it said “Attempt to access Mapping Channel 5 in VertexMap failed. Please be sure the mapping channel exists.” (or similar 
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