If plugin is installed on an older client, known_events might not exist yet.

deluge/plugins/notifications/notifications/common.py

@@ -40,11 +40,16 @@
 import smtplib
 from twisted.internet import defer, threads
 from deluge import component
-from deluge.event import known_events
 from deluge.log import LOG as log
 from deluge.ui.client import client
 import deluge.common
 
+try:
+    from deluge.event import known_events
+except ImportError:
+    # Old deluge version
+    known_events = {}
+
 try:
     import pygame
     SOUND_AVAILABLE = True