Sql select statement returns Null reference error in asp.net

I am view specific category of item in a view. I want it to view all products under that category when clicked but I get a null reference exception. I have seperate tables for the category and products.

Here is what I tried. Code in view:

@model IEnumerable<OpabidFarmsLtd.Models.ViewModels.Shop.CategoryVM>

<h3>Categories</h3>

<ul>
<li><a href="#">All</a></li>
@foreach (var item in Model)
{
    <li><a href="/shop/category/@item.Slug.ToLower()">@item.Name</a></li>
}
 </ul>

And my controller (ShopController):

public ActionResult Category(string name)
    {
        // Declare a list of ProductVM
        List<ProductVM> productVMList;


        using (Db db = new Db())
        {
            // Get category id
            CategoryDTO categoryDTO = db.Categories.Where(x => x.Slug == name).FirstOrDefault();
            int catId = categoryDTO.Id;

            // Init the list
            productVMList = db.Procucts.ToArray().Where(x => x.CategoryId == catId).Select(x => new ProductVM(x))
                .ToList();

            // Get category name
            var productCat = db.Procucts.Where(x => x.CategoryId == catId).FirstOrDefault();
            ViewBag.CategoryName = productCat.CategoryName;
        }

        // Return view with list
        return View(productVMList);
    }

Error Message Screenshot

Edit: While looking for what is wrong, I discovered that the method Category never got the parameter at all, I commented all the codes in it and did this:

 public ActionResult Category(string name)
    {
        string cat;
        using (Db db = new Db())
        {
            CategoryDTO dto = db.Categories.Where(x => x.Slug == "test-category").FirstOrDefault();
            cat = dto.Id.ToString();
        }

        return Content(name + cat);
    }

It returned only 8 which was the Id from the database. I know why it was giving me null, it was because it never received any parameter then default to null. What I can't figure out now is why the parameter isn't getting there from my code above. Please help, I am new to this framework.