Layered materials should not be used with empty 'base material' slot
This message is caused by using CoronaLayeredMTL with empty “Base mtl” slot. Base material is required for the layered material to work properly. If the “Base mtl” slot is empty, a default material will be created with wire (object) color set as diffuse color.
Put a Corona-compatible material in the “Base mtl” slot of offending CoronaLayeredMtl