public ShowroomManager : BeaconManager.IServiceReadyCallback {
    private BeaconManager beaconManager;
    private string scanId;

    public ShowroomManager(Context context) {
        beaconManager = new BeaconManager(context);
        beaconManager.Eddystone += BeaconManager_Eddystone;
    }

    public void StartUpdates() {
        beaconManager.Connect(this); // throws "callback cannot be null" exception
    }

    public void StopUpdates() {
        beaconManager.StopEddyStoneScanning(scanId);
    }

    private void BeaconManager_Eddystone(object sender, BeaconManager.EddystoneEventArgs e) {
        // ...
    }

    // Implementation of IServiceReadyCallback interface:

    public void OnServiceReady() {
        scanId = beaconManager.StartEddystoneScanning();
    }

    public IntPtr Handle { get; }

    public void Dispose() {
        // throw new NotImplementedException();
    }
}