recipe.AddIngredient(null, "angelwings", 1);
recipe.AddIngredient(null, "demonwings", 1);
what you want is
recipe.AddIngredient(ItemID.AngelWings, 1); < - delete the null part, when using vanilla you use an int or an ItemID.
recipe.AddIngredient(ItemID.DemonWings, 1);
if you use ItemID...