make favorites more robust against restarts

qtgui/favorites.py

@@ -84,6 +84,8 @@ class FavoritesTreeController(InstrumentTreeController):
 
         top10 = self.getTop10FavoriteInstruments() # Return an ordered list of double tuple ((libid, instid), counter)
         for placement, ((libId, instId), counter) in enumerate(top10):
+                if not (libId, instId) in self.guiInstruments:
+                    continue
                 gi = self.guiInstruments[(libId, instId)]
                 mostUsed.addChild(gi)
                 gi.injectWidgets() #only possible after gi.init() was done and item inserted.