java.lang.IllegalStateException: Channel closed [channel=c1]. Due to java.lang.NullPointerException: null        at org.apache.flume.channel.file.FileChannel.createTransaction(FileChannel.java:352)        at org.apache.flume.channel.BasicChannelSemantics.getTransaction(BasicChannelSemantics.java:122)        at org.apache.flume.channel.ChannelProcessor.processEventBatch(ChannelProcessor.java:181)        at org.apache.flume.source.SpoolDirectorySource$SpoolDirectoryRunnable.run(SpoolDirectorySource.java:188)        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)        at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317)        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150)        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$101(ScheduledThreadPoolExecutor.java:98)        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.runPeriodic(ScheduledThreadPoolExecutor.java:180)        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:204)        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)        at java.lang.Thread.run(Thread.java:662)Caused by: java.lang.NullPointerException        at org.apache.flume.channel.file.Log.writeCheckpoint(Log.java:1028)        at org.apache.flume.channel.file.Log.replay(Log.java:472)        at org.apache.flume.channel.file.FileChannel.start(FileChannel.java:302)        at org.apache.flume.lifecycle.LifecycleSupervisor$MonitorRunnable.run(LifecycleSupervisor.java:251)        ... 9 more