Object reference not set to an instance of an object for my session

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C#

ASP.NET

C#4.0

Below is the method that loads date in a session

protected void getDayOfWeek(int selectedDay)
        {
           if ((int)DateTime.Now.DayOfWeek >= selectedDay || (int)DateTime.Now.DayOfWeek <= selectedDay)
            {
                Session["SelectedDay"] = DateTime.Now.AddDays(-((int)DateTime.Now.DayOfWeek - selectedDay)).ToShortDateString();
            }
        }

Now below is an event, so i want to use the session above to reload a gridview but it appears that the session is empty for some reasons that i dont know , i get this error "Object reference not set to an instance of an object."

protected void ddlEmployeeID_SelectedIndexChanged(object sender, EventArgs e)
        {
                       DataTable dTable = new DataTable();
                       dTable = systemBusinessLayer.GetAllEmployeesInfo();
            foreach (DataRow dRow in dTable.Rows)//Go through each row inside datatable
            {
                string empIDNumber = dRow["Fullname"].ToString();
                if (ddlEmployeeID.SelectedItem.Text == empIDNumber)
                {
                    Session["idnumber"] = dRow["EmpRecNumber"].ToString();
                                  
               }
              
loagrvSlots(Convert.ToInt32(Session["idnumber"]), Session["SelectedDay"].ToString());
            }
           
        
        }