NullReferenceException on DependencyService

I'm trying to implement Bluetooth on Android, but DependencyService.Get<IBluetooth>().Test() is throwing a NullReferenceException and I cannot figure out why. I think I'm doing everything right. My app is called MyApp. MyApp.Android references MyApp. I have a MyApp.iOS but it is only stubbed out and not set to compile.

MyApp\Home.xaml.cs:

var result = DependencyService.Get<IBluetooth>().Test(); //Debugger fails here, DependencyService.Get() is null.

MyApp\IBluetooth.cs:

namespace MyApp {
    public interface IBluetooth {
        bool Test();
    }
}

MyApp.Android\BluetoothAndroid.cs:

[assembly: Xamarin.Forms.Dependency(typeof(BluetoothAndroid))]
namespace MyApp.Droid {
    public class BluetoothAndroid : IBluetooth {
        bool Test() {
            BluetoothAdapter ba = BluetoothAdapter.DefaultAdapter;
            if (ba == null) return false;
            if (!ba.IsEnabled) return false;
            return true; //Incomplete method, just for testing
        }
    }
}

In my searching around I see lots of warnings to "register" the service but people either never say how or say to write this:

DependencyService.Register<SampleApp.Droid.Services.AddressBook>();

but do not say where that line of code goes. Also, I have not been able to determine if the [assembly] line is a way of registering the service.

My code was originally modeled after Stack Overflow answer /a/44126899/10122542 (I can't post links) about multi-platform toast notifications which actually did work for me briefly but has not in weeks and I cannot fathom why it doesn't. DependencyService.Get<IMessage>().ShortAlert("test") always yields a NullReferenceException!

Where is my code wrong? Thank you for your help!