PluginExtensions.kt

fun Project.android(): BaseExtension {
   val android = project.extensions.findByType(BaseExtension::class.java)
   if (android != null) {
       return android
   } else {
       throw GradleException("Project $name is not an Android project")
   }
}

fun BaseExtension.variants(): DomainObjectSet<out BaseVariant> {
   return when (this) {
       is AppExtension -> {
           applicationVariants
       }

       is FeatureExtension ->{
           featureVariants
       }

       is LibraryExtension -> {
           libraryVariants
       }

       else -> throw GradleException("Unsupported BaseExtension type!")
   }
}