public async Task <ActionResult> DeleteTrainer(int id)
        {
            try {
                await _manager.DeleteTrainer(id);

                return(RedirectToAction(nameof(ShowTrainers)));
            }
            catch (ArgumentNullException) {
                return(RedirectToAction("ErrorPage", nameof(Main), new { message = "Error: can not delete trainer", call = nameof(Trainer) }));
            }
            catch (DbUpdateException) {
                return(RedirectToAction("ErrorPage", nameof(Main), new { message = "Error: invalid input", call = nameof(Trainer) }));
            }
        }