Check if the server is valid during GUILD_MEMBER_REMOVE.

discord/client.py

@@ -276,6 +276,7 @@ class ConnectionState(object):
 
     def handle_guild_member_remove(self, data):
         server = self._get_server(data.get('guild_id'))
+        if server is not None:
             user_id = data['user']['id']
             member = utils.find(lambda m: m.id == user_id, server.members)
             server.members.remove(member)