Object reference not set to an instance of an object

private void GetOldValues()
        {
            ArrayList arr;
            if (Session["SelectedRecords"] != null)
                arr = (ArrayList)Session["SelectedRecords"];
            else
                arr = new ArrayList();
            //Giving me an error "Object reference not set to an instance of an object." 
            CheckBox chkAll = (CheckBox)gvManageJob.HeaderRow
                                 .Cells[0].FindControl("SelectAll");
            for (int i = 0; i < gvManageJob.Rows.Count; i++)
            {
                if (chkAll.Checked)
                {
                    if (!arr.Contains(gvManageJob.DataKeys[i].Value))
                    {
                        arr.Add(gvManageJob.DataKeys[i].Value);
                    }
                }
                else
                {
                    CheckBox chk = (CheckBox)gvManageJob.Rows[i]
                                       .Cells[0].FindControl("chkdelete");
                    if (chk.Checked)
                    {
                        if (!arr.Contains(gvManageJob.DataKeys[i].Value))
                        {
                            arr.Add(gvManageJob.DataKeys[i].Value);
                        }
                    }
                    else
                    {
                        if (arr.Contains(gvManageJob.DataKeys[i].Value))
                        {
                            arr.Remove(gvManageJob.DataKeys[i].Value);
                        }
                    }
                }
            }
            Session["SelectedRecords"] = arr;

        }