ASP.Net MVC Error: There is no ViewData item of type that has the key DisciplineId

hi guys, 

i am new to asp.net mvc, and im coming across this issue and its been days. i have been asked chat GPT to help but, AI cant seem to solve everything. 

System.InvalidOperationException: There is no ViewData item of type IEnumerable<SelectListItem> that has the key DisciplineId.

the system i want right now is like this where, admin is able to edit the discipline in a seperate page. and when they do they, it affects the database. 

likewise, when a user wants to login they can see the discipline table, (which was edited by the admin)

they can pick a discipline, and it will use that to populate a userRole table where there will be userID, roleID, and disicplineID to ensure users have right role,and discpline. that table then is linking to other tables.

public class HomeController : Controller
{
    private readonly EREVIEW_SM_FEEDEntities1 dbContext = new EREVIEW_SM_FEEDEntities1();
    public ActionResult Index()
    {
        //var disciplines = dbContext.disciplines.ToList();
        ////var disciplines = dbContext.disciplines.ToList();  // Assuming 'discipline' is the DbSet name for the discipline table.
        //string selectedName = disciplines[0].disciplineName.ToString();
        //ViewBag.Disciplines = new SelectList(disciplines, "disciplineName", "disciplineName", selectedName);
        //return View();
 
        ViewBag.DisciplineList = new SelectList(dbContext.disciplines, "DisciplineID", "DisciplineName");
        return View();
    }
    public ActionResult LandingPage()
    {
        var comments = dbContext.GetAllComments().ToList();
        return View(comments);
    }
 
    public ActionResult Search()
    {
        // Your logic for the landing page goes here
        return View();
    }
 
    public ActionResult Activity()
    {
        // Your logic for the landing page goes here
        return View();
    }
 
    [HttpPost]
    public ActionResult Register(string FirstName, string LastName, string email, string password, int DisciplineId)
    {
        // Generate salt and hash password
        HashedPassword hpob = new HashedPassword();
        string hashedPassword = hpob.getHashed(password);
        string salt = hpob.getSalt();
 
        try
        {
            // Fetch the discipline name based on DisciplineId
            string disciplineName = dbContext.disciplines
                                            .Where(d => d.disciplineID == DisciplineId)
                                            .Select(d => d.disciplineName)
                                            .FirstOrDefault();
 
            if (string.IsNullOrEmpty(disciplineName))
            {
                ViewBag.ErrorMessage = "Invalid Discipline selected.";
                return View("Index");
            }
 
            // Call the stored procedure with the discipline name
            dbContext.RegisterUser2(FirstName, LastName, email, hashedPassword, salt, disciplineName);
 
            ViewBag.SuccessMessage = "Registration successful!";
        }
        catch (Exception ex)
        {
            string errorMessage = ex.Message;
 
            if (ex.InnerException != null)
                errorMessage += " | Inner Exception: " + ex.InnerException.Message;
 
            ViewBag.ErrorMessage = errorMessage;
        }
 
        return View("Index");
    }
 
    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            dbContext.Dispose();
        }
        base.Dispose(disposing);
    }
}