dgui: Reset DirectOptionMenu highlighted index when replacing items

Not doing this causes issues if the item list is replaced by an item callback, since that means the unhighlight callback is never fired.

Fixes #1125

direct/src/gui/DirectOptionMenu.py

@@ -114,6 +114,7 @@ class DirectOptionMenu(DirectButton):
                                               )
         # Make sure it is on top of all the other gui widgets
         self.popupMenu.setBin('gui-popup', 0)
+        self.highlightedIndex = None
         if not self['items']:
             return
         # Create a new component for each item