How to fix this error null reference in ASP.NET and also put the code of controller?

Question

Why do I get an error and i have also post the code of get method of controller and please check my code is there any error??

public IActionResult upsert(int? id)
    {
        ProductVM productVM = new ProductVM();
        {
           Product product = new Product();
            FlavourSelectList = _db.Flavour.Select(i => new SelectListItem
            {
                Text = i.name,
                Value = i.id.ToString()
            });
        }
    

        if (id==null)
        {

            return View(productVM);
        }
        else
        {
            productVM.Product = _db.Product.Find(id);
            if(productVM.Product == null)
            {
                return NotFound();
            }
            return View(productVM);
        }
        
    }