Asp.net Core MVC adminuser hatası

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using Newtonsoft.Json;
using shopapp.business.Abstract;
using shopapp.entity;
using shopapp.webui.Extensions;
using shopapp.webui.Identity;
using shopapp.webui.Models;

namespace shopapp.webui.Controllers
{

 [Authorize(Roles = "Admin")]
 public class AdminController : Controller.
 {
 private IProductService _productService;
 private ICategoryService _categoryService;

 private RoleManager<IdentityRole> _roleManager;
 private UserManager<User> _userManager;

 public AdminController(IProductService productService, ICategoryService categoryService, RoleManager<IdentityRole> roleManager, UserManager<User> userManager)
 {
 _productService = productService;
 _categoryService = categoryService;
 _roleManager = roleManager;
 _userManager = userManager;
 }

 public async Task<IActionResult> UserEdit(string id)
 {
 var user = await _userManager.FindByIdAsync(id);
 if (user != null)
 {
 var SelectedRoles = await _userManager.GetRolesAsync(user);
 var roles = _roleManager.Roles.Select(i => i.Name);

 ViewBag.Roles = roles;
 return View(new UserDetailModel()
 {
 UserId = user.Id,
 FirstName = user.FirstName,
 LastName = user.LastName,
 UserName = user.UserName,
 Email = user.Email,
 EmailConfirmed = user.EmailConfirmed,
 selectedRoles = SelectedRoles.
 });
 }
 return Redirect("~/admin/user/list");
 }
 [HttpPost]
 public async Task<IActionResult> UserEdit(UserDetailModel model, string[] selectedRoles)
 {
 if (ModelState.IsValid)
 {
 var user = await _userManager.FindByIdAsync(model.UserId);
 if (user != null)
 {
 user.FirstName = model.FirstName;
 user.LastName = model.LastName;
 user.UserName = model.UserName;
 user.Email = model.Email;
 user.EmailConfirmed = model.EmailConfirmed;

 var result = await _userManager.UpdateAsync(user);

 if (result.Succeeded)
 {
 var userRoles = await _userManager.GetRolesAsync(user);
 selectedRoles = selectedRoles ?? new string[] { };
 await _userManager.AddToRolesAsync(user, selectedRoles.Except(userRoles).ToArray<string>());
 await _userManager.RemoveFromRolesAsync(user, userRoles.Except(selectedRoles).ToArray<string>());

 return Redirect("/admin/user/list");
 }
 }
 return Redirect("/admin/user/list");
 }
 return View(model);
 }

 public IActionResult UserList()
 {
 return View(_userManager.Users);
 }

 public async Task<IActionResult> RoleEdit(string id)
 {
 var role = await _roleManager.FindByIdAsync(id);

 var members = new List<User>();
 var nonmembers = new List<User>();

 foreach (var user in _userManager.Users)
 {
 var list = await _userManager.IsInRoleAsync(user, role.Name)
 ? members : nonmembers;
 list.Add(user);

 }
 var model = new RoleDetails()
 {
 Role = role,
 Members = members,
 NonMembers = nonmembers.
 };
 return View(model);
 }

 [HttpPost]
 public async Task<IActionResult> RoleEdit(RoleEditModel model)
 {
 if (ModelState.IsValid)
 {
 foreach (var userId in model.IdsToAdd ?? new string[] { })
 {
 var user = await _userManager.FindByIdAsync(userId);
 if (user != null)
 {
 var result = await _userManager.AddToRoleAsync(user, model.RoleName);
 if (!result.Succeeded)
 {
 foreach (var error in result.Errors)
 {
 ModelState.AddModelError("", error.Description);
 }
 }
 }
 }

 foreach (var userId in model.IdsToDelete ?? new string[] { })
 {
 var user = await _userManager.FindByIdAsync(userId);
 if (user != null)
 {
 var result = await _userManager.RemoveFromRoleAsync(user, model.RoleName);
 if (!result.Succeeded)
 {
 foreach (var error in result.Errors)
 {
 ModelState.AddModelError("", error.Description);
 }
 }
 }
 }
 }
 return Redirect("/admin/role/" + model.RoleId);
 }

 public IActionResult RoleList()
 {
 return View(_roleManager.Roles);
 }

 public IActionResult RoleCreate()
 {
 return View();
 }

 [HttpPost]
 public async Task<IActionResult> RoleCreate(RoleModel model)
 {
 if (ModelState.IsValid)
 {
 var result = await _roleManager.CreateAsync(new IdentityRole(model.Name));
 if (result.Succeeded)
 {
 return RedirectToAction("RoleList");
 }
 else.
 {
 foreach (var error in result.Errors)
 {
 ModelState.AddModelError("", error.Description);
 }
 }
 }
 return View();
 }
 public IActionResult ProductList()
 {
 return View(new ProductListViewModel()
 {
 Products = _productService.GetAll()
 });
 }
 public IActionResult CategoryList()
 {
 return View(new CategoryListViewModel()
 {
 Categories = _categoryService.GetAll()
 });
 }
 [HttpGet]
 public IActionResult ProductCreate()
 {
 return View();
 }

 [HttpPost]
 public IActionResult ProductCreate(ProductModel model)
 {
 if (ModelState.IsValid)
 {
 var entity = new Product()
 {
 Name = model.Name,
 Url = model.Url,
 Price = model.Price,
 Description = model.Description,
 ImageUrl = model.ImageUrl
 };

 if (_productService.Create(entity))
 {
 TempData.Put("message", new AlertMessage()
 {
 Title = "Kayıt eklendi.",
 Message = "Kayıt eklendi.",
 AlertType = "success"
 });

 return RedirectToAction("ProductList");
 }
 TempData.Put("message", new AlertMessage()
 {
 Title = "Hata",
 Message = _productService.ErrorMessage,
 AlertType = "danger"
 });
 return View(model);
 }
 return View(model);

 }

 [HttpGet]
 public IActionResult CategoryCreate()
 {
 return View();
 }

 [HttpPost]
 public IActionResult CategoryCreate(CategoryModel model)
 {
 if (ModelState.IsValid)
 {
 var entity = new Category()
 {
 CategoryId = model.CategoryId,
 Name = model.Name,
 Url = model.Url,
 };
 _categoryService.Create(entity);

 TempData.Put("message", new AlertMessage()
 {
 Title = "Kayıt eklendi.",
 Message = $"{entity.Name} isimli kategori eklendi.",
 AlertType = "success"
 });

 // {"message": "Samsung isimli ürün eklendi.", "AlertType": "danger"}
 return RedirectToAction("CategoryList");
 }
 return View(model);

 }

 [HttpGet]
 public IActionResult ProductEdit(int? id)
 {
 if (id == 0)
 {
 return NotFound();
 }
 var entity = _productService.GetByIdWithCategories((int)id);

 if (entity == null)
 {
 return NotFound();
 }
 var model = new ProductModel()
 {
 ProductId = entity.ProductId,
 Name = entity.Name,
 Price = entity.Price,
 Description = entity.Description,
 Url = entity.Url,
 ImageUrl = entity.ImageUrl,
 IsApproved = entity.IsApproved,
 IsHome = entity.IsHome,
 SelectedCategories = entity.ProductCategories.Select(i => i.Category).ToList()
 };
 ViewBag.Categories = _categoryService.GetAll();
 return View(model);
 }

 [HttpPost]
 public async Task<IActionResult> ProductEdit(ProductModel model, int[] categoryIds, IFormFile file)
 {
 if (ModelState.IsValid)
 {
 var entity = _productService.GetById(model.ProductId);
 if (entity == null)
 {
 return NotFound();
 }
 entity.Name = model.Name;
 entity.Description = model.Description;
 entity.Price = model.Price;
 entity.Url = model.Url;
 entity.IsApproved = model.IsApproved;
 entity.IsHome = model.IsHome;

 if (file != null)
 {
 var extension = Path.GetExtension(file.FileName);
 var randomName = string.Format($"{Guid.NewGuid()}{extension}");
 entity.ImageUrl = randomName;
 var path = Path.Combine(Directory.GetCurrentDirectory(), "wwwroot\\img", randomName);

 using (var stream = new FileStream(path, FileMode.Create))
 {
 await file.CopyToAsync(stream);
 }
 }

 if (_productService.Update(entity, categoryIds))
 {
 TempData.Put("message", new AlertMessage()
 {
 Title = "Kayıt güncellendi.",
 Message = "Kayıt güncellendi.",
 AlertType = "success"
 });

 return RedirectToAction("ProductList");
 }
 TempData.Put("message", new AlertMessage()
 {
 Title = "Hata",
 Message = _productService.ErrorMessage,
 AlertType = "danger"
 });
 CreateMessage(_productService.ErrorMessage, "danger");
 }
 ViewBag.Categories = _categoryService.GetAll();
 return View(model);
 }

 [HttpGet]
 public IActionResult CategoryEdit(int? id)
 {
 if (id == 0)
 {
 return NotFound();
 }
 var entity = _categoryService.GetByIdWithProducts((int)id);

 if (entity == null)
 {
 return NotFound();
 }
 var model = new CategoryModel()
 {
 CategoryId = entity.CategoryId,
 Name = entity.Name,
 Url = entity.Url,
 Products = entity.ProductCategories.Select(p => p.Product).ToList()
 };
 return View(model);
 }

 [HttpPost]
 public IActionResult CategoryEdit(CategoryModel model)
 {
 if (ModelState.IsValid)
 {
 var entity = _categoryService.GetById(model.CategoryId);
 if (entity == null)
 {
 return NotFound();
 }
 entity.Name = model.Name;
 entity.CategoryId = model.CategoryId;
 entity.Url = model.Url;

 _categoryService.Update(entity);

 var msg = new AlertMessage()
 {
 Message = $"{entity.Name} isimli kategori güncellendi.",
 AlertType = "success"
 };
 TempData["message"] = JsonConvert.SerializeObject(msg);

 return RedirectToAction("CategoryList");
 }
 return View(model);

 }

 public IActionResult DeleteProduct(int productId)
 {
 var entity = _productService.GetById(productId);

 if (entity != null)
 {
 _productService.Delete(entity);
 }

 var msg = new AlertMessage()
 {
 Message = $"{entity.Name} isimli ürün silindi.",
 AlertType = "danger"
 };
 TempData["message"] = JsonConvert.SerializeObject(msg);

 return RedirectToAction("ProductList");

 }

 public IActionResult DeleteCategory(int categoryId)
 {
 var entity = _categoryService.GetById(categoryId);

 if (entity != null)
 {
 _categoryService.Delete(entity);
 }

 var msg = new AlertMessage()
 {
 Message = $"{entity.Name} isimli kategori silindi.",
 AlertType = "danger"
 };
 TempData["message"] = JsonConvert.SerializeObject(msg);

 return RedirectToAction("CategoryList");
 }

 [HttpPost]
 public IActionResult DeleteFromCategory(int productId, int categoryId)
 {
 _categoryService.DeleteFromCategory(productId, categoryId);

 return Redirect("/admin/categories/" + categoryId);

 }

 private void CreateMessage(string message, string alerttype)
 {
 var msg = new AlertMessage()
 {
 Message = message,
 AlertType = alerttype.
 };
 TempData["message"] = JsonConvert.SerializeObject(msg);

 }
 }
}

<div class="navbar bg-danger navbar-dark navbar-expand-sm">
  <div class="container">
    <a href="/" class="navbar-brand">ShopApp</a>
    <ul class="navbar-nav mr-auto">
      <li class="nav-item">
        <a href="/products" class="nav-link">Products</a>
      </li>
      @if (User.Identity.IsAuthenticated)
      {
        <li class="nav-item">
          <a href="/cart" class="nav-link">Cart</a>
        </li>
        <li class="nav-item">
          <a href="/orders" class="nav-link">Orders</a>
        </li>

        @if (User.IsInRole("Admin"))
        {
          <li class="nav-item">
            <a href="/admin/products" class="nav-link">Admin Products</a>
          </li>
          <li class="nav-item">
            <a href="/admin/categories" class="nav-link">Admin Categories</a>
          </li>
          <li class="nav-item">
            <a href="/admin/role/list" class="nav-link">Roles</a>
          </li>
          <li class="nav-item">
            <a href="/admin/user/list" class="nav-link">Users</a>
          </li>
        }
      }
    </ul>

    <ul class="navbar-nav ml-auto">
      @if (User.Identity.IsAuthenticated)
      {
        <li class="nav-item">
          <a href="/account/manage" class="nav-link">@User.Identity.Name</a>
        </li>
        <li class="nav-item">
          <a href="/account/logout" class="nav-link">Logout</a>
        </li>
      }
      else
      {
        <li class="nav-item">
          <a href="/account/login" class="nav-link">Login</a>
        </li>
        <li class="nav-item">
          <a href="/account/register" class="nav-link">Register</a>
        </li>
      }
    </ul>


  </div>
</div>