c# – Asp.Net Core 2.0 ArgumentNullException：值不能为null.

我是第一次使用数据库,我遇到了以下问题：

unhandled exception occurred while processing the request.
ArgumentNullException: Value cannot be null.
Parameter name: connectionString.

在阅读并尝试每一个可能的建议以及可能的解决方案之后,问

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {    
        services.AddMvc();
        services.AddDbContext<VideosContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    }

    public void Configure(IApplicationBuilder app,IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseBrowserLink();
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Home/Error");
        }

        app.UseStaticFiles();

        app.UseMvc(routes =>
        {
            routes.MapRoute(
name: "default",template: "{controller=Home}/{action=Index}/{id?}");
        });
    }
}
    {
      "ConnectionString": {
        "DefaultConnection": "Server =(localdb)mssqllocaldb;Database=Videos;Trusted_Connection=True;MultipleActiveResultSets=true"
    },"Logging": {
      "IncludeScopes": false,"LogLevel": {
        "Default": "Warning"
      }
    }
}

public static void Main(string[] args)
{
     BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args) =>
   WebHost.CreateDefaultBuilder(args)
      .UseStartup<Startup>()
      .Build();

我究竟做错了什么？提前感谢您的任何建议.