null check for writer if not initialized yet during syncrunner run

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

@@ -594,9 +594,11 @@ public class FSHLog extends AbstractFSWAL<Writer> {
           long start = System.nanoTime();
           Throwable lastException = null;
           try {
+            if(writer != null) {
               TraceUtil.addTimelineAnnotation("syncing writer");
               writer.sync(sf.isForceSync());
               TraceUtil.addTimelineAnnotation("writer synced");
+            }
             currentSequence = updateHighestSyncedSequence(currentSequence);
           } catch (IOException e) {
             LOG.error("Error syncing, request close of WAL", e);