fix for:

http://www.gramps-project.org/bugs/view.php?id=5960
http://www.gramps-project.org/bugs/view.php?id=5958
errors when a person is unknown in the database.


svn: r20170

src/plugins/lib/libsubstkeyword.py

@@ -856,6 +856,8 @@ class VariableParse(object):
     
     def get_event_by_name(self, person, event_name):
         """ get an event from a name. """
+        if not person:
+            return None
         for e_ref in person.get_event_ref_list():
             if not e_ref:
                 continue