Error message displays when try to access Sitemap in dashboard

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I upgraded from 5.3 to 5.4 last week but my css wasn't loading so I downgraded again to a version that was working properly.

My problem is that now I cannot access the sitemap, I get this message:
Fatal error: Call to undefined method ConcreteDashboardSitemapHelper::outputRequestHTML() in /home/cimepr/public_html/concrete/single_pages/dashboard/sitemap/full.php on line 19

Can anyone explain & help me get it sorted. This is a client site so I want to make sure it's working OK before I let them at the back end. I'd prefer not to have to delete the DB & install again from scratch but if needs must I will.

PassionForCreative