ASP.NET MVC EF Code First Many-to-Many Insert

Hello,

I am using EF Code First approach for my web application. I have two entities, Service Providers and Categories and a many-to-many relationship between them. I managed the lookup table automatically generated by using the following code, but could not manage to insert into the lookup table. I'm getting a null reference error when using the following code. I am looking forward to hearing a solution:

 

In ServiceProvidersController:

        [AcceptVerbs("POST")]
        public ActionResult Create(ServiceProvider newSP)
        {
            if (ModelState.IsValid)
            {
                var categoryId = Request["CategoryID"];
                var catIds = categoryId.Split(',');
                foreach (var catId in catIds)
                {
                    int id = int.Parse(catId);

                    Category category = db.Categories.SingleOrDefault(d => d.CategoryID == id);
                    newSP.Categories.Add(category);
                }
                db.ServiceProviders.Add(newSP);
                db.SaveChanges();

                return RedirectToAction("Index");
            }
 
            return View();
        }

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public class Category
    {
        public int CategoryID { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }

        public ICollection<ServiceProvider> ServiceProviders { get; set; }
    }

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public class ServiceProvider
    {
        public int ServiceProviderID { get; set; }
        [Required]
        public string Title { get; set; }
        public string Telephone { get; set; }
        public string Fax { get; set; }
        public string Address { get; set; }
        
        public virtual ICollection<Category> Categories { get; set; }       
    }

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