session becoming automatically null

Question

getting below error : 

An exception of type 'System.NullReferenceException' occurred in App_Code.xdxzpwdw.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

 

description :

 

below code working fine in other classes but in one class session becommig null automatically.

I am passing post request using ajax and uploading image and deleting image also same time.

Kindly check below code. I did not understand why this problem is occuringexatly. Kindly help.

 

code details : 

public object BL_product_InsertUpdate(tbl_product product)//BL_admin_userInsertUpdate

{

   string message = string.Empty;

   if (!string.IsNullOrEmpty(HttpContext.Current.Session["UserName"].ToString()))

   {

   }

 }