Configuration.AddJsonFile("config.json") NullReferenceException - asp.net-core-mvc

Question

I take NullReferenceException when I want to add addJsonFile. I made exaclty what lynda(http://www.lynda.com/ASP-NET-tutorials/Dynamically-control-behavior-custom-configuration/368051/431234-4.html) said.
(screenShot)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNet.Builder;
using Microsoft.AspNet.Hosting;
using Microsoft.AspNet.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.AspNet.Diagnostics;
using Microsoft.Framework.Internal;
using Microsoft.Framework.ConfigurationModel;
using Microsoft.Framework.ConfigurationModel.Json;
namespace PortalDemo
{
public class Startup
{
// This method gets called by the runtime. Use this method to add services to the container.
// For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
public void ConfigureServices(IServiceCollection services)
{
services.AddMvc();
}
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app)
{
var config = new Configuration();
config.AddEnvironmentVariables();
config.AddJsonFile("config.json");//Here
if (config.Get("debug") == "True")
{
app.UseRuntimeInfoPage();
app.UseDeveloperExceptionPage();
}
app.UseExceptionHandler("/home/errorPage");
app.UseMvc(routes=>
routes.MapRoute("Default","{controller=Home}/{action=Index}/{id?}"));
app.UseStaticFiles();
}
// Entry point for the application.
public static void Main(string[] args) => WebApplication.Run<Startup>(args);
}
}

Answer 1

Try the following code.
public class Startup
{
public Startup(IApplicationEnvironment appEnv)
{
IConfigurationBuilder builder = new ConfigurationBuilder()
.SetBasePath(appEnv.ApplicationBasePath)
.AddJsonFile("config.json", false);
Configuration = builder.Build();
}
public IConfiguration Configuration { get; set; }
}