c# android Attempt to invoke virtual method 'android.content.ContentResolver android.content.Context.getContentResolver()' on a null object reference

Question

I have an imageview, and a button, starting an image intent.

I pass the uri from image intent to a function that should return the real path from the uri.

here's the activity result of image intent:

protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
{
    base.OnActivityResult(requestCode, resultCode, data);

    FindViewById<TextView>(Resource.Id.imguri1).Text = a.GetRealPathFromURI(data.Data);
}

and the function returning the real path:

public class GiocaUri : Activity
{
    public string GetRealPathFromURI(Android.Net.Uri a)
    {
        ICursor cursor;
        cursor = ContentResolver.Query(a, null, null, null, null);
        cursor.MoveToFirst();
        string documentId = cursor.GetString(0);
        documentId = documentId.Split(':')[1];
        cursor.Close();

        cursor = ContentResolver.Query(
            Android.Provider.MediaStore.Images.Media.ExternalContentUri,
            null, MediaStore.Images.Media.InterfaceConsts.Id + " = ? ", 
            new[] { documentId }, null);
        cursor.MoveToFirst();
        string path = cursor.GetString(
            cursor.GetColumnIndex(MediaStore.Images.Media.InterfaceConsts.Data));
        cursor.Close();

        return path;

    }
}

I test the app and after choosing image, i get error here:

ContentResolver.Query(a, null, null, null, null);

this exception:

Unhandled Exception:

Java.Lang.NullPointerException: Attempt to invoke virtual method 'android.content.ContentResolver android.content.Context.getContentResolver()' on a null object reference

I wrote uri in console and it's not null.

What i'm doing wrong? Thanks!

Edit: As far as I know, there is nothing NULL (well.. contenresolver.query needs only one value, others can bee NULL, i tried filling them with a projector and so on, obviously not null things, and problem was the same).

I understand why this post could be a duplicate, but I wrote variables in console and are not null.

Plus, I used this guide for image intent, and this for resolver.

Plus, I tried another method to query the database, always from second link, and the error is the same:

string path = null;

            string[] projection = new[] 
            { Android.Provider.MediaStore.Images.Media.InterfaceConsts.Data };

            var loader = new CursorLoader(this, a, projection, null, null, null);

            var cursor = (ICursor)loader.LoadInBackground();
            if (cursor.MoveToFirst())
            {
                path = cursor.GetString(cursor.GetColumnIndex(
                    MediaStore.Images.Media.InterfaceConsts.Data));
            }
            cursor.Close();

                return path;

error is in

var loader = new CursorLoader(this, a, projection, null, null, null);

Don't mark this as duplicate please.