当我执行Flink应用程序时，它会给我以下信息NullPointerException：
2017-08-08 13:21:57,690 INFO com.datastax.driver.core.Cluster - New Cassandra host /127.0.0.1:9042 added
2017-08-08 13:22:02,427 INFO org.apache.flink.runtime.taskmanager.Task - TriggerWindow(TumblingEventTimeWindows(30000), ListStateDescriptor{serializer=org.apache.flink.api.common.typeutils.base.ListSerializer@15d1c80b}, EventTimeTrigger(), WindowedStream.apply(CoGroupedStreams.java:302)) -> Filter -> Flat Map -> Sink: Cassandra Sink (1/1) (092a7ef50209f7a050d9d82be1e03d80) switched from RUNNING to FAILED.
java.lang.RuntimeException: Exception occurred while processing valve output watermark:
at org.apache.flink.streaming.runtime.io.StreamInputProcessor$ForwardingValveOutputHandler.handleWatermark(StreamInputProcessor.java:289)
at org.apache.flink.streaming.runtime.streamstatus.StatusWatermarkValve.findAndOutputNewMinWatermarkAcrossAlignedChannels(StatusWatermarkValve.java:173)
at org.apache.flink.streaming.runtime.streamstatus.StatusWatermarkValve.inputWatermark(StatusWatermarkValve.java:108)
at org.apache.flink.streaming.runtime.io.StreamInputProcessor.processInput(StreamInputProcessor.java:188)
at org.apache.flink.streaming.runtime.tasks.OneInputStreamTask.run(OneInputStreamTask.java:69)
at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:263)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:702)
at java.lang.Thread.run(Thread.java:748)
Caused by: org.apache.flink.streaming.runtime.tasks.ExceptionInChainedOperatorException: Could not forward element to next operator
at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.pushToOperator(OperatorChain.java:530)
at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.collect(OperatorChain.java:503)
at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.collect(OperatorChain.java:483)
at org.apache.flink.streaming.api.operators.AbstractStreamOperator$CountingOutput.collect(AbstractStreamOperator.java:891) …