parent: don't send messages on streams that no longer exist; closes #480

mitogen/parent.py

@@ -1677,6 +1677,10 @@ class RouteMonitor(object):
         :param str name:
             Context name or :data:`None`.
         """
+        if not stream:
+            # We may not have a stream during shutdown.
+            return
+
         data = str(target_id)
         if name:
             data = '%s:%s' % (target_id, name)