adding await ystore start method in prepare method

jupyter_collaboration/handlers.py

@@ -111,6 +111,7 @@ def exception_logger(exception: Exception, log: Logger) -> bool:
                     file = self._file_loaders[file_id]
                     updates_file_path = f".{file_type}:{file_id}.y"
                     ystore = self._ystore_class(path=updates_file_path, log=self.log)
+                    await ystore.start()
                     self.room = DocumentRoom(
                         self._room_id,
                         file_format,