IdFTP : OnWorkBegin/OnWork Générate Excepti on

Hello,

I try to show a progress bar for the file that i upload
But it generate an exception

Access violation at address 078D0577. writing adress 00000000


Sample code :

jvgpFile is a JVCL Componant TJvGradientProgressBar

procedure TfrmFTP.IdFTP1WorkBegin(ASender: TObject;
   AWorkMode: TWorkMode; AWorkCountMax: Int64);
begin
   if AWorkMode = wmWrite then
   begin
     jvgpFile.Max := AWorkCountMax; // <- Exception here
     Application.ProcessMessages;
   end;
end;

procedure TfrmFTP.IdFTP1Work(ASender: TObject; AWorkMode: TWorkMode;
   AWorkCount: Int64);
begin
   if AWorkMode = wmWrite then
   begin
     jvgpFile.Position := AWorkCount;
     Application.ProcessMessages;
   end;
end;

Thanks.