从appsettings.json获取值并返回NullReferenceException：对象引用未设置为对象的实例。在.net core 3中

不知道我在这里缺少什么，但是我没有从我appsettings.json的.NET Core应用程序中获取值。我已经读了几乎所有关于appsettings.json归国的文章NullReferenceException。我知道有重复的帖子，但是他们没有解决问题

我已经完成了我所知道的设置，也可以在线阅读，但仍然返回null。我已经重构了几次代码，但仍然遇到了

NullReferenceException：对象引用未设置为对象的实例。

Startup.cs +中的ShopAPI.Startup.ConfigureServices（IServiceCollection服务）

var key = Encoding.UTF8.GetBytes（Configuration [“ ApplicationSettings：JWT_Secret”]。ToString（））;

System.RuntimeMethodHandle.InvokeMethod（对象目标，对象[]参数，签名sig，布尔构造函数，布尔wrapExceptions）

Startup 类：

public IConfiguration Configuration { get; }

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
      //.Get<Dictionary<string, string>>()

        services.AddOptions();
        var applicationSettings = Configuration.GetSection("AppSetting");
        services.Configure<ApplicationSettings>(applicationSettings);

        //services.AddSingleton(Configuration.GetSection("AppSettings").Get<ApplicationSettings>());

        services.AddControllers();

        services.AddDbContext<ShopDbContext>(option =>
        option.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

        services.AddIdentity<ApplicationUser,IdentityRole>()
           .AddEntityFrameworkStores<ShopDbContext>()
           .AddDefaultTokenProviders();

        services.Configure<IdentityOptions>(options => {
            options.Password.RequireDigit = false;
            options.Password.RequireUppercase = false;
            options.Password.RequireNonAlphanumeric = false;
            options.Password.RequiredLength = 6;
        });

        services.AddCors();

        //Jwt Authentication configuration

        //var key = Encoding.UTF8.GetBytes(Configuration["ApplicationSettings: JWT_Secret"].ToString());
        var key = Encoding.UTF8.GetBytes(Configuration["AppSetting : JWT_Secret"].ToString());

        services.AddAuthentication(x => {
            x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
            x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
            x.DefaultScheme = JwtBearerDefaults.AuthenticationScheme;
        }).AddJwtBearer(x =>
        {
            x.RequireHttpsMetadata = false;
            x.SaveToken = false;
            x.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuerSigningKey = true,
                IssuerSigningKey = new SymmetricSecurityKey(key),
                ValidateIssuer = false,
                ValidateAudience = false,
                ClockSkew = TimeSpan.Zero
            };
        });
}

appsettings.json

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=api2;Trusted_Connection=True;Connection Timeout=120;MultipleActiveResultSets=true"
  },
  "AppSetting": {
    "JWT_Secret": "1234567890123456",
    "Client_URL": "http://localhost:4500"
  },
  "AllowedHosts": "*"
}