Some users of my application experience the exception below when closing a NUIDocument that has an NForm for its client.  The NForm has a CommandBarsManager on it.  Seems to be a timing issue as it doesn't happen consistently so I can't provide a test program to demonstrate this.  I'm using build 13.11.26.12.  Could you change your get_Visible() method to check for NULL before using its objects?

System.NullReferenceException: Object reference not set to an instance of an object.
   at Nevron.UI.WinForm.Controls.NCommandState.get_Visible()
   at Nevron.UI.WinForm.Controls.NCommandParent.OnPaint(PaintEventArgs e)
   at System.Windows.Forms.Control.PaintWithErrorHandling(PaintEventArgs e, Int16 layer)
   at System.Windows.Forms.Control.WmPaint(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at Nevron.UI.WinForm.Controls.NCommandParent.WndProc(Message& m)
   at System.Windows.Forms.Nativewindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)