Catch KeyError on ModelData.get_id in default method

Since changeset 685f2d888dca, the default methods are always called in
__register__ when the XML files are not yet loaded.

issue6409

stock.py

@@ -68,7 +68,10 @@ class ConfigurationSequence:
     def default_package_sequence(cls):
         pool = Pool()
         ModelData = pool.get('ir.model.data')
-        return ModelData.get_id('stock_package', 'sequence_package')
+        try:
+            return ModelData.get_id('stock_package', 'sequence_package')
+        except KeyError:
+            return None
 
 
 class Package(ModelSQL, ModelView):