Add missing model on board

(transplanted from f201401a91f7b300b1036e4c23d2153f2af791f2)

tryton/gui/window/board.py

@@ -28,6 +28,7 @@ class Board(SignalEvent):
             self.name = self.board.name
         else:
             self.name = name
+        self.model = ''
 
         self.widget = gtk.VBox()