Fix location domain if warehouse is empty

location.py

@@ -17,7 +17,7 @@ class ProductLocation(ModelSQL, ModelView):
             domain=[('type', '=', 'warehouse')])
     location = fields.Many2One('stock.location', 'Storage Location',
             required=True, domain="[('type', '=', 'storage'), " \
-                "('parent', 'child_of', [warehouse])]")
+                "('parent', 'child_of', warehouse and [warehouse] or [])]")
     sequence = fields.Integer('Sequence')
 
     def __init__(self):