When setting the old face set custom data type, do it in the "layers to write" vector instead of the mesh's poly custom data layers, which weren't "prepared" yet. Otherwise this could make the mesh's custom data typemap invalid.
When setting the old face set custom data type, do it in the "layers to write" vector instead of the mesh's poly custom data layers, which weren't "prepared" yet. Otherwise this could make the mesh's custom data typemap invalid.