Fix decoding logic for /rooms.

src/modules/room/RoomsPane.tsx

@@ -20,8 +20,14 @@ interface UrlParams {
 
 const RoomChatPane: FC<ChatProps> = ({ loginUserName, rooms }) => {
   const { roomId } = useParams<UrlParams>();
-  const name = decode(roomId);
-  return <RoomChat loginUserName={loginUserName} room={rooms[name]} />;
+
+  let room;
+  if (roomId !== undefined) {
+    const name = decode(roomId);
+    room = rooms[name];
+  }
+
+  return <RoomChat loginUserName={loginUserName} room={room} />;
 };
 
 const RoomsPane: FC<{}> = () => {