Error when get connection string: ArgumentNullException: Value cannot be null. Parameter name: connectionString

I am using ASP.NET Core 2.0. Below is my code.

Startup:

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

        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)
        {
            // Add framework services.
            services
                .AddMvc()
                .AddJsonOptions(options => options.SerializerSettings.ContractResolver = new DefaultContractResolver());
            services.AddDbContext<fooContext>(options => options.UseSqlServer(Configuration.GetConnectionString("UserDatabase")));
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        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?}");
            });
        }
    }
}

appsettings.json:

{
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    },
    "ConnectionStrings": {
      "UserDatabase": "Server=DESKTOP-FSES7UK;Database=xxx;User Id=sa; Password=xxxxxxx;Trusted_Connection=True;"
    }
  }
}

How to fix it?