Asp.net mvc core System.NullReferenceException: 'Object reference not set to an instance of an object.' exception

Question

I am developing e commerce project with asp.net core 6 mvc. My controller;

[HttpGet]
public async Task<IActionResult> ProductListing(int categoryId)
{
    var products = await _productService.TGetByCategoryId(categoryId);

    List<ProductViewModel> productList = new List<ProductViewModel>();

    foreach (var item in products)
    {
        ProductViewModel model = new ProductViewModel();
        model.ProductId = item.Id;
        model.Name = item.Name;
        model.Price = item.Price;
        model.ImageUrl = item.ImageUrl;
        productList.Add(model);

    }  
    
    var category = await _categoryService.TGetById(categoryId);   //in this line I had System.NullReferenceException: 'Object reference not set to an instance of an object.' exception.
    //ViewBag.CategoryName = category.CategoryName;
    
    return View(productList);
}

How can I fix it ?

I am expecting there is no exception, I couldnt get how this include the exception.