MVC: Bind null reference when attempting to register the IProductsRepository with the DI container

gistfile1

using Ninject;
using Ninject.Modules;
using SportsStore.Domain.Abstract;
using SportsStore.Domain.Concrete;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace SportsStore.WebUI.Infrastructure
{
    /// <summary>
    /// Stop MVC from instantiating controller classes directly, & make it request them from your DI container
    /// This will allow the DI container to resolve any dependencies those controllers may have.  
    /// </summary>
    
    public class NinjectControllerFactory : DefaultControllerFactory
    {
        // A Ninject "kernel" is the thing that can supply object instances
        private IKernel kernel = new StandardKernel(new SportsStoreServices());

        // ASP.NET MVC calls this to get the controller for each request
        protected override IController GetControllerInstance(RequestContext context, Type controllerType)
        {
            if (controllerType == null)
                return null;
            return (IController)kernel.Get(controllerType);
        }

        // Configure how abstract service types are mapped to concrete implementations
        private class SportsStoreServices : NinjectModule
        {
            public override void Load()
            {
                Bind<IProductsRepository>()
                    .To<SqlProductsRepository>()
                    .WithConstructorArgument("connectionString",
                                             ConfigurationManager.ConnectionStrings["AppDb"].ConnectionString
                    );
            }
        }
    }
}