C#

(System.NullReferenceException: Object reference not set to an instance)

private void SaveData()
{
try
{
SYS_WS sysws = new SYS_WS();
sysws.SaveWindowCompleted +=new SaveWindowCompletedEventHandler(sysws_SaveWindowCompleted);
sysws.SaveWindowAsync((SYSDataset)dsSystem.GetChanges());
}
catch (Exception ex)
{
throw (ex);
}
}

private void sysws_SaveWindowCompleted(object sender,
SaveWindowCompletedEventArgs e)
{
try
{
if (e.Error != null)
{
return;
}
this.dsSystem.AcceptChanges();

}
catch (Exception ex)
{
throw ex;
}
}

[WebMethod(EnableSession = true)]
public bool SaveWindow(SYSDataset ds)
{
try
{
WindowController cont = new WindowController();
cont.SaveWindows(ds);
return true;
}
catch (Exception ex)
{
throw (ex);
}
}


[WebMethod(EnableSession = true)]
public SYSDataset LoadWindow(string WindowID)
{
WindowController cont = new WindowController();
return cont.LoadWindow(WindowID);
}
---- Stack Trace ----
   RTR.SYS_V.Setup.frmWindow.SaveData()
       frmWindow.cs: line 0091, col 17, IL 0079
   RTR.SYS_V.Setup.frmWindow.btnSave_Click(sender As Object, e As EventArgs)
       frmWindow.cs: line 0202, col 13, IL 0001
   System.Windows.Forms.Control.OnClick(e As EventArgs)
       MAIN.EXE: N 00111
   System.Windows.Forms.Button.OnClick(e As EventArgs)
       MAIN.EXE: N 00073
   System.Windows.Forms.Button.OnMouseUp(mevent As MouseEventArgs)
       MAIN.EXE: N 00171
   System.Windows.Forms.Control.WmMouseUp(m As Message&, button As MouseButtons, clicks As Int32)
       MAIN.EXE: N 00654
   System.Windows.Forms.Control.WndProc(m As Message&)
       MAIN.EXE: N 8788613
   System.Windows.Forms.ButtonBase.WndProc(m As Message&)
       MAIN.EXE: N 8807204
   System.Windows.Forms.Button.WndProc(m As Message&)
       MAIN.EXE: N 00031
   System.Windows.Forms.ControlNativeWindow.OnMessage(m As Message&)
       MAIN.EXE: N 00015
   System.Windows.Forms.ControlNativeWindow.WndProc(m As Message&)
       MAIN.EXE: N 00048
   System.Windows.Forms.NativeWindow.Callback(hWnd As IntPtr, msg As Int32, wparam As IntPtr, lparam As IntPtr)
       MAIN.EXE: N 00089