我试图在两个Dstream上做左外连接，两者都是[k：String，V：GenericData.Record]类型，但是收到错误
User class threw exception: org.apache.spark.SparkException: Job aborted due to stage failure: Task 2.0 in stage 0.0 (TID 2) had a not serializable result: org.apache.avro.generic.GenericData$Record