Robotics.Mobile.Core.Bluetooth.LE Adapter is null

Hi all,
i am new in Xamarin an i tried to use Bluetooth in my Xamarin.Forms project. I have looked in the sample code from Monkey.Robotics project and have done the same.
But when i try
static IAdapter Adapter;
in App.cs, the Adapter is null and a exeption occurs when i use the Adapter.
Have anyone an idea?

Here is my code:

App.cs :

`using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Xamarin.Forms;
using Robotics.Mobile.Core.Bluetooth.LE;

namespace AwigoWaageXamarinForms
{
public class App : Application
{
static IAdapter Adapter;

    public App()
    {
        // The root page of your application
        MainPage = new NavigationPage(new AwigoWaageXamarinForms.MainPage(Adapter));
    }

    public static void SetAdapter (IAdapter adapter)
    {
        Adapter = adapter;
    }

   ...
}

}`

MainPage.xaml.cs :

`using System;
using System.Collections.Generic;
using Xamarin.Forms;

using Robotics.Mobile.Core.Bluetooth.LE;
using System.Threading.Tasks;
using System.Diagnostics;
using System.Collections.ObjectModel;

namespace AwigoWaageXamarinForms
{
public partial class MainPage : ContentPage
{

    IAdapter adapter;
    ObservableCollection<IDevice> devices;

    public MainPage (IAdapter adapter)
    {
        InitializeComponent ();

        this.adapter = adapter;
        this.devices = new ObservableCollection<IDevice> ();
        listView.ItemsSource = devices;

        // !!!!!!!!  here occurs the Exeption, because adapter is null !!!!!!!!!!!!!!!!!!
        adapter.DeviceDiscovered += (object sender, DeviceDiscoveredEventArgs e) => {
            Device.BeginInvokeOnMainThread(() => {
                devices.Add (e.Device);
            });
        };

        adapter.ScanTimeoutElapsed += (sender, e) => {
            adapter.StopScanningForDevices(); // not sure why it doesn't stop already, if the timeout elapses... or is this a fake timeout we made?
            Device.BeginInvokeOnMainThread ( () => {
                DisplayAlert("Timeout", "Bluetooth scan timeout elapsed", "OK", "");
            });
        };
    }

...
`