Issue in a controller of an ASP.NET Core MVC app

Question

I don't know what the problem is. My data is being saved, but toast message and redirection to an action method are not working. The fun part is - while debugging, it executes both ending conditions but I don't know why this is not working! Toast messages and redirections are working on other controllers.

Code:

[HttpPost]
public async Task<IActionResult> CreateSlot(AppointmentViewModel model, Appointment appointment)
{
    var currentUser = await _tokenService.GetUserViaToken();

    string[] timeParts = model.Time.Split('-');

    if (timeParts.Length == 2)
    {
        string startTimeString = timeParts[0].Trim();
        string endTimeString = timeParts[1].Trim();

        // only start time
        TimeOnly startTime = TimeOnly.ParseExact(startTimeString, "hh:mm tt", CultureInfo.InvariantCulture);
        TimeOnly endTime = TimeOnly.ParseExact(endTimeString, "hh:mm tt", CultureInfo.InvariantCulture);

        var appointmentDate = DateTime.ParseExact(model.Date, "yyyy-MM-dd", CultureInfo.InvariantCulture);

        var dateCheck = appointmentDate.DayOfWeek.ToString();

        var slotId = _context.AvailableSlots
                             .Where(x => x.UserId == model.DoctorId 
                                         && x.StartTime == startTime 
                                         && x.EndTime == endTime 
                                         && x.Day.Name == appointmentDate.DayOfWeek.ToString())
                             .Select(x => x.Id)
                             .FirstOrDefault();

        var data = new Appointment
          {
              AppointmentById = currentUser.Id,
              AppointmentWithId = model.DoctorId,
              CreatedTime = DateTime.UtcNow,
              AppointmentDate = appointmentDate,
              AppointmentStatusId = 1,
              AvailableSlotId = slotId
          };

        _context.Appointments.Add(data);
        await _context.SaveChangesAsync();

        TempData.AddToastMessage("success", "Appointment Created!");

        return RedirectToAction("Index", "Home");
    }
    else
    {
        return null;
    }
}

I want to show the the toast message and redirect to the Index view