1. Build the attached project (or any other project which only has a single icon layer set).
2. Try to build the proejct in Xcode, you'll get a similiar error:

".../UnityImages.xcassets: The image stack "App Icon - Small" must have at least 2 layers with applicable content. Although it has 2 layers, only 1 has applicable content."

There should be some warning/error that you have to set both layers when building.