dbus-binding: Fix 'pop' error when no validator given

Fixes openbmc/openbmc#606.

Change-Id: Id39435f8177d053c317ebe2e48c1c3fd9d3a2385
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/obmc/dbuslib/bindings.py b/obmc/dbuslib/bindings.py
index d3fdbaf..f426482 100644
--- a/obmc/dbuslib/bindings.py
+++ b/obmc/dbuslib/bindings.py
@@ -31,7 +31,7 @@
 
 class DbusProperties(dbus.service.Object):
     def __init__(self, **kw):
-        self.validator = kw.pop('validator')
+        self.validator = kw.pop('validator', None)
         super(DbusProperties, self).__init__(**kw)
         self.properties = {}
         self._export = False