Update EvtSelector._active_period to remove the exception.

modules/plugin_event/selector.py

@@ -206,7 +206,7 @@ class EvtSelector(SelectorActiveItems):
             return (rows[0][0], rows[-1][1])
 
         else:
-            raise StandardError("Failed to find start / leave date !")
+            return (None, None)
 
     def _age(self, row):
         """Compute the age of the person associated to the history record, now.