Object reference not set to an instance of an object

ContentControl control;
            string selectedItem = string.Empty;
            int selectedCategory = 0;
            string selectedPrice = string.Empty;

            // Controls for Category ComboBox
            control = categoryComboBox.SelectedItem as ContentControl;
            if (control != null)
            {
                selectedItem = (string)control.Content;
            }

            // Controls for Price ComboBox
            control = priceComboBox.SelectedItem as ContentControl;
            if (control != null)
            {
                selectedPrice = (string)control.Content;
            }

            #region
            if (selectedItem == "Electronics and Computers")            // Entertainment Option
            {
                selectedCategory = 1;
            }
            else if (selectedItem == "TV and Video")            // Entertainment Option
            {
                selectedCategory = 2;
            }
            else if (selectedItem == "Home Audio and Theater")            // Entertainment Option
            {
                selectedCategory = 3;
            }
            else if (selectedItem == "Camera, Photo and Video")            // Entertainment Option
            {
                selectedCategory = 4;
            }
            else if (selectedItem == "Cell Phones and Accessories")            // Entertainment Option
            {
                selectedCategory = 5;
            }
            else if (selectedItem == "Video Games")            // Entertainment Option
            {
                selectedCategory = 6;
            }
            else if (selectedItem == "MP3 Players and Portable Speakers")            // Entertainment Option
            {
                selectedCategory = 7;
            }
            else if (selectedItem == "Car Electronics and GPS")            // Entertainment Option
            {
                selectedCategory = 8;
            }
            else if (selectedItem == "Musical Instruments")            // Entertainment Option
            {
                selectedCategory = 9;
            }
            else if (selectedItem == "Electronics Accessories")            // Entertainment Option
            {
                selectedCategory = 10;
            }
            else if (selectedItem == "Laptops and Tablets")            // Entertainment Option
            {
                selectedCategory = 11;
            }
            else if (selectedItem == "Desktops & Monitors")            // Entertainment Option
            {
                selectedCategory = 12;
            }
            else if (selectedItem == "Computer Accessories and Peripherals")            // Entertainment Option
            {
                selectedCategory = 13;
            }
            else if (selectedItem == "Computer Parts and Components")            // Entertainment Option
            {
                selectedCategory = 14;
            }
            else if (selectedItem == "Software")            // Entertainment Option
            {
                selectedCategory = 15;
            }
            else if (selectedItem == "PC Games")            // Entertainment Option
            {
                selectedCategory = 16;
            }
            else if (selectedItem == "Musical Instruments")            // Entertainment Option
            {
                selectedCategory = 9;
            }
            #endregion



            var query = context.CreateQuery<Customer>("GetProducts").AddQueryOption("selectedCategory", selectedCategory);
                                                                //    .AddQueryOption("selectedPrice", selectedPrice);

            productsViewSource.Source = query.Expand("ProductOptions");

            productsDataGrid.SetBinding(DataGrid.ItemsSourceProperty, "ProductOptions");

[WebGet]
        public IQueryable<Product> GetProducts(int selectedCategory)
        {
            if (selectedCategory > 0)
            {
                iShopEntities context = this.CurrentDataSource;
                var products = from product in context.Products    // .Include("ProductOptions")
                               where product.CategoryID == selectedCategory
                           //    && product.Price == selectedPrice
                               select product;

                return products;
            }
            else
            {
                throw new DataServiceException("Blah");
            }
        }