You need to do some swapping around
Agk.Update(0);
Agk.ClearScreen();
Agk.Swap();
// put command here of what you want to render here - there is nothing here - so your rendering nothing here at moment
// is that true?- if not then what would you like to render to your picture file? a.jpeg?
Agk.Render();
Agk.GetImage(1, 0, 0, 1920, 1080);
Agk.SaveImage(1, "raw:/AGK/output/media/a.jpeg");
Agk.DeleteImage(1);
EDIT - Are you wishing to take a screen shot and save the current state of your app to a picture file - to potentially create a save/load thumbnail??? Nice idea....
Tier 1 Developer