code: private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
  		in.defaultReadObject();
  
  		// the nested serializer will be null if this was read from a savepoint taken with versions
  		// lower than Flink 1.7; in this case, we explicitly create instance for the nested serializer.
  		if (versionSerializer == null || nodeIdSerializer == null || eventIdSerializer == null) {
  			this.versionSerializer = DeweyNumber.DeweyNumberSerializer.INSTANCE;
  			this.eventIdSerializer = EventId.EventIdSerializer.INSTANCE;
  			this.nodeIdSerializer = new NodeId.NodeIdSerializer();
  		}
  	}
docs: /*
  Backwards compatible deserializing of NFAStateSerializer.