最近在練習登入及註冊功能

我是參考這部影片

https://youtu.be/R5Zw3fn2YE4?t=1977

用的是內建的Identity機制

我有在Table AspNetUsers 裡追加兩個欄位

 public class ApplicationUser : IdentityUser
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }

    }

最終畫面是這樣

這是相關的Controller

  [HttpPost]
        public async Task<IActionResult> Create(ApplicationUser user)
        {
            if (ModelState.IsValid)
            {
                var result = await _userManager.CreateAsync(user, user.PasswordHash);
                if (result.Succeeded)
                {
                    var isSaveRole = await _userManager.AddToRoleAsync(user,"User");
                    TempData["save"] = "User has been created successfully";
                    return RedirectToAction(nameof(Index));
                }
                foreach (var error in result.Errors)
                {
                    ModelState.AddModelError(string.Empty,error.Description);
                }
            }


            return View();
        }

只是每次一跑到isSaveRole這邊時就會出錯，最後就呈現標題那樣的錯誤
想請問我是錯在哪裡呢?

這是我的GIT
https://github.com/Jarkwoof/ShoppingCart

路徑是Customer/User/Create

拜託了，謝謝