问"ArgumentNullException:值不能为null。Arg_ParamName_Name“在ASP.NET核心中突然出现

EN

public class SummonerDataInitializer
{
    private readonly SummonerContext context;
    private readonly UserManager<User> userManager;
    private readonly SummonerRepository summonerRepository;

    public SummonerDataInitializer(SummonerContext context, UserManager<User> userManager, SummonerRepository summonerRepository)
    {
        this.context = context;
        this.userManager = userManager;
        this.summonerRepository = summonerRepository;
    }

    public async Task InitializeData()
    {
        await context.Database.EnsureDeletedAsync();
        if (await context.Database.EnsureCreatedAsync())
        {
            var dashboard = new Dashboard
            {
                PinnedSummoners = new List<Summoner>
                {
                    await summonerRepository.GetBy("euw", "Jhinstachio"),
                    await summonerRepository.GetBy("euw", "DracoZar"),
                    await summonerRepository.GetBy("euw", "TrueMrCrazy"),
                    await summonerRepository.GetBy("euw", "God of Lunar")
                },
                LiveGameItems = new List<LiveGameItem>
                {
                    new LiveGameItem
                    {
                        Region = "euw",
                        SummonerName = "Don Arts",
                    },
                    new LiveGameItem
                    {
                        Region = "euw",
                        SummonerName = "3250",
                    },
                    new LiveGameItem
                    {
                        Region = "euw",
                        SummonerName = "Cboi1",
                    },
                }
            };

            var user = new User("admin", "admin@admin.com") { EmailConfirmed = true };
            await userManager.CreateAsync(user, "P@ssword1");
            await userManager.AddClaimAsync(user, new Claim(ClaimTypes.Role, "admin"));
            
            user = await context.Users.SingleAsync(u => u.Email == user.Email);
            user.Dashboard = dashboard; // Removing this line fixes the exception
            context.Users.Update(user);
            await context.SaveChangesAsync();
            
        }
    }
}