[Flush] NPE when region flushs

2019-07-09 08:02:14,262 FATAL org.apache.hadoop.hbase.regionserver.HRegionServer: ABORTING region server hostname,16020,1562233574704: Replay of WAL required. Forcing server shutdown
org.apache.hadoop.hbase.DroppedSnapshotException: region: namespace:table,963,1562296120996.b8e2f19748d374d192b93f106a0f73b3.
        at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushCacheAndCommit(HRegion.java:2646)
        at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2322)
        at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2284)
        at org.apache.hadoop.hbase.regionserver.HRegion.flushcache(HRegion.java:2170)
        at org.apache.hadoop.hbase.regionserver.HRegion.flush(HRegion.java:2095)
        at org.apache.hadoop.hbase.regionserver.MemStoreFlusher.flushRegion(MemStoreFlusher.java:508)
        at org.apache.hadoop.hbase.regionserver.MemStoreFlusher.flushRegion(MemStoreFlusher.java:478)
        at org.apache.hadoop.hbase.regionserver.MemStoreFlusher.access$900(MemStoreFlusher.java:76)
        at org.apache.hadoop.hbase.regionserver.MemStoreFlusher$FlushHandler.run(MemStoreFlusher.java:264)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
        at java.util.ArrayList.<init>(ArrayList.java:177)
        at org.apache.hadoop.hbase.regionserver.StoreScanner.updateReaders(StoreScanner.java:863)
        at org.apache.hadoop.hbase.regionserver.HStore.notifyChangedReadersObservers(HStore.java:1172)
        at org.apache.hadoop.hbase.regionserver.HStore.updateStorefiles(HStore.java:1145)
        at org.apache.hadoop.hbase.regionserver.HStore.access$900(HStore.java:122)
        at org.apache.hadoop.hbase.regionserver.HStore$StoreFlusherImpl.commit(HStore.java:2505)
        at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushCacheAndCommit(HRegion.java:2600)
        ... 9 more