NULLReferenceException for a selected drop-down value in an asp.net core mvc application using Mailkit

I am working on an asp.net core 5.0 MVC application, I have used Mailkit Nuget package to send an email to the client after creating a record with all its answers. I have put the code related to the Mailkit inside the Create(Post) method but after creating and sending email I am getting System.NullReferenceException in my Mailkit code part for one of my fields called Product Type when I write it like sale.ProductType.ProductTypeDesc which is basically a drop-down and has a look-up table and so a foreign key to the main table(Sale). but it is working with sale.ProductTypeId which is not something that I want. Can anyone give me solution?

Here are my models:

using System;

using System.Collections.Generic;

#nullable disable

namespace Sale_app.Models

{

public partial class Sale

{
    public int SaleId { get; set; }

    public string ProductName { get; set; }

    public string ProductTypeId { get; set; }
    

    public virtual ProductType ProductType { get; set; }
 
}

}

using System;

using System.Collections.Generic;

#nullable disable

namespace Sale_app.Models

{ public partial class ProductType

{
    public ProductType()

    {

        Sales = new HashSet<Sale>();

    }

    public int ProductTypeId { get; set; }

    public string ProductTypeDesc { get; set; }

    public virtual ICollection<Sale> Sales { get; set; }

}

}

and these are Create methods in my Controller:

//Get Sale

public IActionResult Create()

    {

        ViewBag.ProductTypeList = _context.ProductType.ToList();

        return View();
    }

[HttpPost]

    [ValidateAntiForgeryToken]

    public IActionResult Create([Bind("SaleId,ProductTypeId,ProductName")] Sale sale)

    {
        
        if (ModelState.IsValid)
        {
            try
            {
                _context.Add(sale);
                _context.SaveChanges();

                //instantiate a new MimeMessage
                var message = new MimeMessage();
                //Setting the To e-mail address
                message.To.Add(new MailboxAddress("E-mail Recipient Name", "an email address comes here"))
                //Setting the From e-mail address
                message.From.Add(new MailboxAddress("Sale Form " + "Response", "an email address comes here"));
                //E-mail subject 
                message.Subject = "Sale Form";
                //E-mail message body
                message.Body = new TextPart(TextFormat.Html)
                {
                    Text = "<b>SaleId:</b> " + sale.SaleId + "<br/>" + "<b>ProductName:</b> " + sale.ProductName + "<br/>" +
                    "<b>ProductType</b>: " + sale.ProductType.ProductTypeDecs

                };

                //Configure the e-mail
                using (var emailClient = new SmtpClient())
                {
                    emailClient.Connect("smtp.gmail.com", 587, false);
                    emailClient.Authenticate("an email address comes here", "a password comes here");
                    emailClient.Send(message);
                    emailClient.Disconnect(true);
                }
                return RedirectToAction(nameof(Index));

            }

            catch (Exception ex)
            {
                ModelState.Clear();
                ViewBag.Message = $" Oops! We have a problem here {ex.Message}";
            }
        }


        return View(sale);
       
    }