private void SaveField(object fieldInstance)
        {
            FieldViewModel fieldViewModel = fieldInstance as FieldViewModel;

            if (fieldViewModel.Field != null)
            {
                if (!fieldViewModel.IsValid)
                {
                    Application.Current.MainPage.DisplayAlert("Предупреждение", "Задайте название поля", "Ok");
                    return;
                }

                using (AppDbContext db = App.GetContext())
                {
                    if (fieldViewModel.Field.FieldId == 0)
                    {
                        db.Entry(fieldViewModel.Field).State = EntityState.Added;
                    }
                    else
                    {
                        db.Fields.Update(fieldViewModel.Field);
                    }
                    db.SaveChanges();
                }
            }
            Back();
        }
        private void DeleteField(object fieldInstance)
        {
            FieldViewModel fieldViewModel = fieldInstance as FieldViewModel;

            if (fieldViewModel.Field != null && fieldViewModel.Field.FieldId != 0)
            {
                using (AppDbContext db = App.GetContext())
                {
                    db.Fields.Remove(fieldViewModel.Field);
                    db.SaveChanges();
                }
            }
            Back();
        }