This message is triggered when using a Corona Layered Material or Corona Light Override Material with an empty “Base mtl” slot. A base material is required for the layered materials to work properly. If the “Base mtl” slot is empty, a default material will be created with the wire (object) color set as its diffuse color.
Solution: Add a Corona-compatible material to the “Base mtl” slot of the offending Corona Layered Material or Corona Light Override Material.