How to add Curtainwall Grid..? help me?

i Create CurtainWall in API..

my code is...

Wall wall = Wall.Create(RequestHandler.m_uidoc.Document, m_CurveList, s.Id, level.Id, true, nomal);

and i want to get CurtainWall Grid and Add Curtainwallgrid.

CurtainGridLine cgline = RequestHandler.m_uidoc.Document.GetElement(fi.Id) as CurtainGridLine; <-- null; -_-;;

CurtainGrid cgrid = fi.CurtainGrid; 

XYZ pp = m_CurtainwallGridList[0].GetEndPoint(0);

cgline = cgrid.AddGridLine(false, pp, false);  <-- error

what`s wrong?