HBASE-15976 RegionServerMetricsWrapperRunnable will be failure when disable blockcache. - Addendum

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerWrapperImpl.java

@@ -317,6 +317,9 @@ class MetricsRegionServerWrapperImpl
 
   @Override
   public long getBlockCacheFailedInsertions() {
+    if (this.cacheStats == null) {
+      return 0;
+    }
     return this.cacheStats.getFailedInserts();
   }