[To dev/1.3] Fix potential NPE in Unseq List #16360

Author: JackieTien97

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/DataRegion.java

@@ -2093,6 +2093,21 @@ private boolean tryGetFLushLock(
               isDebug)) {
         TsFileProcessor tsFileProcessor = tsFileResource.getProcessor();
         try {
+          if (tsFileProcessor == null) {
+            // tsFileProcessor == null means this tsfile is being closed, here we try to busy loop
+            // until status in TsFileResource has been changed which is supposed to be the last step
+            // of closing
+            while (!tsFileResource.isClosed() && waitTimeInMs > 0) {
+              TimeUnit.MILLISECONDS.sleep(5);
+              waitTimeInMs -= 5;
+            }
+            if (tsFileResource.isClosed()) {
+              continue;
+            } else {
+              clearAlreadyLockedList(needToUnLockList);
+              return false;
+            }
+          }
           long startTime = System.nanoTime();
           if (tsFileProcessor.tryReadLock(waitTimeInMs)) {
             // minus already consumed time