The flush table operation encountered an NPE and failed.

Executing a merge region before the flush table is completed will result in an NPE error for the flush table and cause it to fail

Triggering logic

1. Execute the flush table command.
2. Merge regions before the flush is completed.
3. After the merge is completed, the two parent regions will be cleaned up.
4. An NPE will be reported when the flush operation is performed on that parent region.