Some users have been reporting an exception like this:

System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
bei Infragistics.Win.ToolTip.set_Font(Font value)
bei Infragistics.Win.ToolTip.OnFormDisposed(Object sender, EventArgs e)
bei System.ComponentModel.Component.Dispose(Boolean disposing)
bei System.Windows.Forms.Control.Dispose(Boolean disposing)
bei System.Windows.Forms.Form.Dispose(Boolean disposing)
bei System.Windows.Forms.Form.WmClose(Message& m)
bei Infragistics.Win.ToolTipFormEx.WndProc(Message& m)
bei System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Alas, I can't reproduce it on my end, making this a bit hard to track down.

This seems to happen when my application is in the process of closing.

Users reporting it use a version of my app with Infragistics 21.1.20211.62. In contrast, an older version of my app has 16.2.20162.2182, and that one is apparently not affected.

The exception also seems to be occurring no the main/UI thread, so it's probably not a threading issue.

I haven't tried installing the latest service release nor upgrading to 21.2, because I've thus far been unable to reproduce this on my own. Since there's none of my code in the stacktrace, I'm unsure how to proceed further. Any idea how to narrow this down? Is it perhaps fixed in a service release?