ProductRepository Throwing Null Reference Exception

 private readonly IRepository<Product> _productVendorRepository;

public VendorService(IRepository<Product> productVendorRepository)
        {
            this._productVendorRepository = productVendorRepository;
            
            this._vendorService = EngineContext.Current.Resolve<IVendorService>();
            this._cacheManager = EngineContext.Current.Resolve<ICacheManager>();
       }

 public virtual IList<VendorOrder> getVendorSoppingCartItems(IList<ShoppingCartItem> shoppingCartItems)
        {
            var vendorOrders = new List<VendorOrder>();
          

            foreach (var item in shoppingCartItems)
            {
           

                var query = from ps in _productVendorRepository.Table
                          where ps.Id == item.Product.Id
                            select ps.Vendor;
                var vendor = query.FirstOrDefault();


                int index = vendorOrders.FindIndex(s => s.vendor == vendor);
                if (index >= 0)
                {
                    vendorOrders[index].ShoppingCartItems.Add(item);
                }
                else
                {
                    var newVendorOrder = new VendorOrder();
                    newVendorOrder.vendor = vendor;
                    newVendorOrder.ShoppingCartItems.Add(item);
                    vendorOrders.Add(newVendorOrder);
                }
            }
            return vendorOrders;
        }

                var query = from ps in _productVendorRepository.Table
                          where ps.Id == item.Product.Id
                            select ps.Vendor;
                var vendor = query.FirstOrDefault();

                var query = from ps in _productVendorRepository.Table
                          where ps.Id == item.Product.Id
                            select ps.Vendor;
                var vendor = query.FirstOrDefault();

item.product.Id

if(item.product != null) {
var query = from ps in _productVendorRepository.Table
                          where ps.Id == item.Product.Id
                            select ps.Vendor;
}

if(query != null) {
 var vendor = query.FirstOrDefault();
}