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blazor ranking app #3을 하던 도중에 스크린샷과 같은 에러가 나타났습니다.

콘솔창에는 아래와 같은 에러 메세지가 떠서 제 입력이 userId가 null인 것으로 입력된 것 같습니다.

문제가 있을 수 있어 하단에 제가 작성한 ranking.razor와 RankingService.cs의 코드를 첨부하지만 강의의 내용에서 작성한 코드와 동일하게 작성하였습니다.

ranking.razor

@page "/ranking"
@using RankingApp.Data.Models
@using RankingApp.Data.Services

@inject RankingService RankingService
<h3>Ranking</h3>
@*LoginDisplay.razor의 내용 참조*@
<AuthorizeView>
    <Authorized> @*로그인한 경우*@
        @if (_gameResults == null)
        {
            <p><em>Loading...</em></p>
        }
        else
        {
            <table class="table">
                <thead>
                    <tr>
                        <th>User</th>
                        <th>Score</th>
                        <th>Date</th>
                    </tr>
                </thead>
                <tbody>
                    @foreach (var gameResult in _gameResults)
                    {
                        <tr>
                            <td>@gameResult.UserName</td>
                            <td>@gameResult.Score</td>
                            <td>@gameResult.Date.ToString()</td>
                        </tr>
                    }
                </tbody>
            </table>

            @*데이터 추가하기*@
            <p>
                <button class="btn btn-primary" @onclick="AddGameResult">
                    Add
                </button>
            </p>

            @if (_showPopup)
            {
                <div class="modal" style="display:block" role="dialog">
                    <div class="modal-dialog">
                        <div class="modal-content">
                            <div class="modal-Header">
                                <h3 class="modal-title">Add/Update GameResult</h3>
                                <button type="button" class="close" @onclick="ClosePopup">
                                    <span area-hidden="true">X</span>
                                </button>
                            </div>
                            <div class="modal-body">
                                <label for="UserName">UserName</label>
                                <input class="form-control" type="text" placeholder="UserName" @bind-value="_gameResult.UserName">
                                <label for="Score">Score</label>
                                <input class="form-control" type="text" placeholder="Score" @bind-value="_gameResult.Score">
                                <button class="btn btn-primary" @onclick="SaveGameResult">
                                    Save
                                </button>
                            </div>
                        </div>
                    </div>
                </div>
            }
        }
    </Authorized>
    <NotAuthorized>
        <p>You are not Authorized</p>
    </NotAuthorized>
</AuthorizeView>


@code {
    List<GameResult> _gameResults;

    bool _showPopup;
    GameResult _gameResult;

    protected override async Task OnInitializedAsync()
    {
        _gameResults = await RankingService.GetGameResultsAsync();
    }

    void AddGameResult()
    {
        @*팝업 띄워주기*@
        _showPopup = true;
        _gameResult = new GameResult() { Id = 0 };

    }

    void ClosePopup()
    {
        _showPopup = false;
    }

    async Task SaveGameResult()
    {
        if(_gameResult.Id==0) //새로 데이터를 추가하는 작업, 이때는 id가 없지만 db에 넣어지면id 자동 생성
        {
            _gameResult.Date=DateTime.Now;
            var result = RankingService.AddGameResult(_gameResult); @*AddGameResult는 rankingService.cs에 있다*@
        }
        else
        {
            //TODO
        }
        _gameResults = await RankingService.GetGameResultsAsync(); @*다시 바뀐 데이터 다시 가져오기*@
    }
}

RankingService.cs

using RankingApp.Data.Models;

namespace RankingApp.Data.Services
{
    public class RankingService
    {
        ApplicationDbContext _context;
        public RankingService(ApplicationDbContext context)
        {
            _context = context;
        }

        //Create
        public Task<GameResult> AddGameResult(GameResult gameResult)
        {
            _context.GameResults.Add(gameResult);
            _context.SaveChanges(); //db에도 저장

            return Task.FromResult(gameResult);
        }
        //Read
        public Task<List<GameResult>> GetGameResultsAsync()
        {
            List<GameResult> results = _context.GameResults
                .OrderByDescending(item => item.Score)
                .ToList();

            return Task.FromResult(results);
        }

        //Update
        //Delete
    }
}