触发wwDBGridListTitleButtonClick事件时出现 invalid pointer operation 错误

代码如下:procedure TE_frmEDIBooking01.wwDBGridListTitleButtonClick(Sender: TObject;
  AFieldName: String);
var
  smainSql,sSql:string;
begin
   try
     qryBillList.Close;
     smainSql:='Select * From b_booking '
         +' where vslcode="'+Edit_vslcode.Text+'" and voycode="'+Edit_voy.Text+'" and confirmSign<>"Q" ';
     if bState=true then
        sSql:=smainSql+' order by '+AFieldName+' desc '
     else
        sSql:=smainSql+' order by '+AFieldName+' asc ';     qryBillList.SQL.Text:=sSql;
     qryBillList.Open;
     wwDBGridlist.UnselectAll;
     bState:=not bState;
   except
     showmessage('SORRY!排序过程中出现错误!');
   end;
end;
执行到最后一行end时就出错.
   qryBillList.open时触发了qryBillListAfterScroll()

代码如下:
   procedure TE_frmEDIBooking01.qryBillListAfterScroll(DataSet: TDataSet);
var
  sBillID:String;
begin
  if IsModified then begin
     FPubBDE.ShowInfo('资料已经修改,请先保存或撤销！');
     Exit;
    end;
  sBillID:=qryBillList.FieldByName('booking_no').AsString;
  LoadBill(sBillID);
end;请大家帮忙看看.谢谢