What version are you running?
Review Board/rb-site 6.0
ReviewBoardPowerPack 5.2.2
From the Review Board 6.0 docker image
What steps will reproduce the problem?

rb-site manage <site> export-db

This is a rather older site that has been migrated up version-by-version
rb-site manage <site> import-db


What is the expected output? What do you see instead?
Expected would be a completed import.  Instead, a partial import and the error described in command output below.
What operating system are you using?
Linux -- Review Board 6.0 docker image
Attach the debug out from the command.

$ rbt manage --debug /site import-db -- -b /tmp/reviewboard-export/bundle/ -k /tmp/reviewboard-export/key --user-strategy=merge --review-group-strategy=rename --repository-strategy=merge
DEBUG:root:Logging to /site/logs/reviewboard.log with a minimum level of DEBUG
DEBUG:/venv/lib/python3.11/site-packages/rbpowerpack/scmtools/versionvault.pyc - Using cleartool cleartool
DEBUG:reviewboard.scmtools.svn:Using reviewboard.scmtools.svn.pysvn backend for SVN
DEBUG:reviewboard.scmtools.clearcase:Using cleartool cleartool
Importing 2799 review requests...
Traceback (most recent call last):
  File "/venv/bin/rb-site", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/venv/lib/python3.11/site-packages/reviewboard/cmdline/rbsite.py", line 3329, in main
    command.run(site, options)
  File "/venv/lib/python3.11/site-packages/reviewboard/cmdline/rbsite.py", line 3099, in run
    site.run_manage_command(manage_command, manage_args)
  File "/venv/lib/python3.11/site-packages/reviewboard/cmdline/rbsite.py", line 1225, in run_manage_command
    manage_util.execute()
  File "/venv/lib/python3.11/site-packages/django/core/management/__init.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/venv/lib/python3.11/site-packages/django/core/management/base.py", line 354, in run_from_argv
    output = self.execute(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "build/bdist.macosx-13.5-arm64/wheel/rbpowerpack/importexport/management/commands/import-db.py", line 213, in handle
  File "/venv/lib/python3.11/site-packages/six.py", line 718, in reraise
    raise value.with_traceback(tb)
rbpowerpack.importexport.errors.ImportDatabaseError: The reference from `FileDiff.commit` to `DiffCommit` is required, but is empty! Please report this.



Please provide any additional information below.
This data was originally hosted on a 1-series reviewboard and migrated through 2 eventualy up to 6.

The import-db fails on 6.0 and also the 5.0.x versions I have tried.

The ultimate goal is to merge this reviewboard data into another instance.