We're getting a fairly regular crash when using the UltraComboEditor.  The error seems to happen when you select the combo box and then hit letter keys to select items in the list.  We have DropDownStyle set to DropDownList and DropDownListWidth set to -1, other than that we took all the defaults.  Here is the stack trace that we're getting:
 
System.NullReferenceException: Object reference not set to an instance of an object.
   at Infragistics.Win.ValueListItem.DoesTextMatch(String testString, Boolean partial)
   at Infragistics.Win.ValueListItemsCollection.GetItemFromDisplayText(Int32& lastValueListIndex, String displayText, Int32 beginAtIndex, Boolean partial)
   at Infragistics.Win.ValueList.Infragistics.Win.IValueList.GetValue(String text, Int32& index, Int32 beginAtIndex, Boolean partial)
   at Infragistics.Win.EditorWithCombo.ProcessAlphaNumericChar(Char keyChar)
   at Infragistics.Win.EditorWithCombo.OnOwnerKeyPress(KeyPressEventArgs e)
   at Infragistics.Win.EmbeddableEditorBase.ProcessOwnerKeyPress(Object sender, KeyPressEventArgs e)
   at Infragistics.Win.EmbeddableEditorOwnerBase.RaiseKeyPress(KeyPressEventArgs e)
   at Infragistics.Win.UltraWinEditors.UltraComboEditor.OnKeyPress(KeyPressEventArgs e)
   at System.Windows.Forms.Control.ProcessKeyEventArgs(Message& m)
   at System.Windows.Forms.Control.ProcessKeyMessage(Message& m)
   at System.Windows.Forms.Control.WmKeyChar(Message& m)
   at System.Windows.Forms.Control.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)