Исключение System.NullReferenceException при вызове через Android.Net.Sip

using System;
 
using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using Android.Net.Sip;
 
namespace AndroidApplication1
{
    [Activity(Label = "AndroidApplication1", MainLauncher = true, Icon = "@drawable/icon")]
    public class Activity1 : Activity
    {
        public SipManager manager = null;
        public SipProfile me = null;
        public SipAudioCall call = null;
 
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
 
            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);
 
            // Get our button from the layout resource,
            // and attach an event to it
            Button button = FindViewById<Button>(Resource.Id.MyButton);
 
            button.Click += delegate
            {
                manager = SipManager.NewInstance(this);
                String username = "user";
                String password = "12345";
                String domain = "myserver.net";
                SipProfile.Builder builder = new SipProfile.Builder(username, domain);
                builder.SetPassword(password);
                me = builder.Build();
                manager.Open(me);
            };
        }
    }
}

SipProfile.Builder builder = new SipProfile.Builder(username, domain);
builder.SetPassword(password);
me = builder.Build();
manager.Open(me);

manager.Open(me);