I suppose once of the latest builds introduced this exception. This happens every time on server shutdown after all dimensions get saved. IIRC, there were some NightConfig-related commits recently in ModernFix, hope this will help.
P.S. Providing a screenshot as this exception is not logged neither in latest.log, nor in debug.log

Thanks, I've fixed this for the next release.

Should be fixed as of 5.4.1.