Error display after click submit button Object reference not set to an

I working on MVC razor pages asp.net core 7 . I can't display success message after submit and insert data on same page .

I get error

System.NullReferenceException: 'Object reference not set to an instance of an object.'

error happen on Model.userModel.Branches

Expected Result is display success message after insert data success but after click submit button

data inserted and give me this error null reference exception

so How to solve this error ?

public class AddUserModel : PageModel
    {
        [BindProperty]
        public AddUserViewModel userModel { get; set; }
        [BindProperty]
        public UC.ADC.Core.Entities.SQL.User  User { get; set; }
        public string SuccessMessage { get; set; }
public AddUserModel(ADCContext db,OracleContext ora_db2)
        {
            
             _db = db;
            userModel = new AddUserViewModel();
        }
     public async Task OnPost()
            {
                if (ModelState.IsValid)
                {
                    await _db.User.AddAsync(User);
                    await _db.SaveChangesAsync();
                    SuccessMessage = "Form submitted successfully!";
                    return Page();
                    
    
                }
                
            }
    public void OnGet()
            {
                userModel.Branches  = _db.Branch.ToList();
             }
  }

on adduser.cshtml

@page "/AddUser"

       

        
            Branch

public class AddUserModel : PageModel { [BindProperty] public AddUserViewModel userModel { get; set; } [BindProperty] public UC.ADC.Core.Entities.SQL.User User { get; set; } public string SuccessMessage { get; set; } public AddUserModel(ADCContext db,OracleContext ora_db2) { _db = db; userModel = new AddUserViewModel(); userModel.Branches= new List(); // change 1 } public async Task OnPost() { if (ModelState.IsValid) { await _db.User.AddAsync(User); await _db.SaveChangesAsync(); SuccessMessage = "Form submitted successfully!"; return Page(); } } public void OnGet() { userModel.Branches = _db.Branch.ToList(); } }