An exception error

public ActionResult Details(int? id)

{

var dbo = new DrugBusiness();

DrugModel dm = dbo.DetailsMethod(id);

DateTime drug_expdate = dbo.GetAll().Find(x => x.DrugId == id).ExpiryDate;

DateTime now = DateTime.Today;

var days = drug_expdate - now;

if(drug_expdate<=now)

{

ViewBag.msg = "The Drug has expired";

}

else

{

ViewBag.msg = "This Drug expires in " + days.Days + "Days";

}

return PartialView(dbo.DetailsMethod(id));

 

but when im running my Details view it's giving me an error in yellow line, this what it says "{"Object reference not set to an instance of an object."}" 

please help me ive try but nothing is working, please