Hi I am facing a very simple problem but I am not exactly sure why?
I am trying to call directly from xamarin forms app, without showing dailer screen, user will see list of its contacts in a screen click a button to call, and it will make a direct call without showing dialer screen.
to achieve this I have used DependencyServic and I have used this as my base https://www.c-sharpcorner.com/UploadFile/e4bad6/code-to-start-call-in-xamarin-forms/
the only difference is this is PCL and I am using shared library
Where I am getting Problem?

My Interface

public interface IPhoneCall
    {
        void MakeQuickCall(string PhoneNumber);
    }

My call to Dependency Service

private void makeCall_Clicked(object sender, EventArgs e)
        {
            try
            {
                DependencyService.Get<IPhoneCall>().MakeQuickCall("+xxxxxxxxx");
            } catch(Exception ex)
            {
                DisplayAlert("Alert!!!", ex.Message, "ok");
            }
        }

My Dependency service call for Android:

[assembly: Dependency(typeof(PhoneCall_Droid))]
namespace MakeCall.Droid
{
    public class PhoneCall_Droid : IPhoneCall
    {
        public void MakeQuickCall(string PhoneNumber)
        {
            try
            {
                var uri = Android.Net.Uri.Parse(string.Format("tel:{0}", PhoneNumber));
                var intent = new Intent(Intent.ActionCall, uri);
                Xamarin.Forms.Forms.Context.StartActivity(intent);
            }
            catch (Exception ex)
            {
                new AlertDialog.Builder(Android.App.Application.Context).SetPositiveButton("OK", (sender, args) =>
                {
                    //User pressed OK
                })
                .SetMessage(ex.ToString())
                .SetTitle("Android Exception")
                .Show();
            }
        }

    }
}

This alert is returning an exception Unable to add window - token null is not valid; is your activity running?
I have looked different solutions like this
Intent ActionCall is not making phone call in xamarin forms
and this https://forums.xamarin.com/discussion/129166/unable-to-add-window-token-null-is-not-for-an-application-alertbuilder
but I am still not able to sort this, out,
also I tried this in my Main activity

internal static MainActivity Instance { get; private set; }

and then added this line in OnCreate method

Instance = this;

and changed my Android dependency service class method to this

public void MakeQuickCall(string PhoneNumber)
        {
            var context = MainActivity.Instance;
            try
            {

                new AlertDialog.Builder(context ).SetPositiveButton("OK", (sender, args) =>
                {
                    //User pressed OK
                })
                .SetMessage(Android.Net.Uri.Parse(string.Format("tel:{0}", PhoneNumber)).ToString())
                .SetTitle("Android Exception")
                .Show();


                Intent intent = new Intent(Intent.ActionCall, Android.Net.Uri.Parse(string.Format("tel:{0}", PhoneNumber)));
                context.StartActivity(intent);
            }
            catch (Exception ex)
            {
                new AlertDialog.Builder(Android.App.Application.Context).SetPositiveButton("OK", (sender, args) =>
                {
                    //User pressed OK
                })
                .SetMessage(ex.ToString())
                .SetTitle("Android Exception")
                .Show();
            }
        }

and by doing this alert is showing the dailed phone number, but it is not calling and for call part is still showing same of unable to add window ..,