Is 'List' unsupported in Widgets?

The following code, in an independent project works fine:

Code Block struct ContentView: View {
    var body: some View {
        List {
            Text("Hello, world!")
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Code Block
struct ContentView: View {

var body: some View {

List {

Text("Hello, world!")
}
}
}

struct ContentView_Previews: PreviewProvider {

static var previews: some View {

ContentView()

}
}

The following code in a Widget Extension results in an error being displayed in the SwiftUI view (it builds fine):

Code Block struct ContentView: View {
    var body: some View {
        List {
            Text("Hello, World!")
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .previewContext(WidgetPreviewContext(family: .systemMedium))
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().previewContext(WidgetPreviewContext(family: .systemMedium))
    }
}