Fix YRoomManager initialization (#32)

Co-authored-by: Naresh Kumar Kolloju <[email protected]>

Author: knaresh

jupyter_rtc_core/rooms/yroom_manager.py

@@ -28,6 +28,7 @@ def __init__(
         self.contents_manager = contents_manager
         self.loop = loop
         self.log = log
+        self._rooms_by_id = {}
         # Initialize dictionary of YRooms, keyed by room ID