I've written a simple test app to replicate this issue (attached).

The main form has UltraDockManager on it and it has two dock panes when startup. I close down these two panes, and I move focus to another windows application, e.g., IE. When i focus back to the main form, I got the following exception.

System.NullReferenceException: Object reference not set to an instance of an object.
   at Infragistics.Win.UltraWinDock.DockableGroupPane.set_SelectedTabPane(DockablePaneBase value)
   at Infragistics.Win.UltraWinDock.DockablePaneBase.set_IsSelectedTab(Boolean value)
   at Infragistics.Win.UltraWinDock.UltraDockManager.NotifyPossibleActivePaneChange()
   at Infragistics.Win.UltraWinDock.UltraDockManager.OnFormActivated(Object sender, EventArgs e)
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Windows.Forms.Form.OnActivated(EventArgs e)
   at System.Windows.Forms.Form.set_Active(Boolean value)
   at System.Windows.Forms.Form.WmActivate(Message& m)
   at System.Windows.Forms.Form.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

It looks like a bug. Any suggestion on fixing it?

Thanks.