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

purchase.py

@@ -72,8 +72,11 @@ class PurchaseConfigPurchaseDropLocation(ModelSQL, ValueMixin):
     def default_purchase_drop_location(cls):
         pool = Pool()
         ModelData = pool.get('ir.model.data')
-        return ModelData.get_id(
-            'sale_supply_drop_shipment', 'location_drop')
+        try:
+            return ModelData.get_id(
+                'sale_supply_drop_shipment', 'location_drop')
+        except KeyError:
+            return None
 
 
 class Purchase:

sale.py

@@ -52,8 +52,11 @@ class SaleConfigSaleDropLocation(ModelSQL, ValueMixin):
     def default_sale_drop_location(cls):
         pool = Pool()
         ModelData = pool.get('ir.model.data')
-        return ModelData.get_id(
-            'sale_supply_drop_shipment', 'location_drop')
+        try:
+            return ModelData.get_id(
+                'sale_supply_drop_shipment', 'location_drop')
+        except KeyError:
+            return None
 
 
 class Sale:

stock.py

@@ -77,8 +77,11 @@ class ConfigurationSequence:
     def default_shipment_drop_sequence(cls):
         pool = Pool()
         ModelData = pool.get('ir.model.data')
-        return ModelData.get_id(
-            'sale_supply_drop_shipment', 'sequence_shipment_drop')
+        try:
+            return ModelData.get_id(
+                'sale_supply_drop_shipment', 'sequence_shipment_drop')
+        except KeyError:
+            return None
 
 
 class ShipmentDrop(Workflow, ModelSQL, ModelView):