I got this message with the OAI-PMH harvester activated. This error appears both when trying to delete any item. I had harvested from a site with some malformed XML. The error stops when I de-activate OAI-PMH Harvester.
Fatal error: Call to a member function delete() on a non-object in /Library/WebServer/Documents/omeka/plugins/OaipmhHarvester/plugin.php on line 200
Thanks for testing the harvester and finding this bug. It will be fixed in the stable release.
The 1.0 stable version of the OAI-PMH harvester, which fixes the bug you reported, has been released.
You can upgrade from the 1.0 release candidate to the 1.0 stable release without uninstalling the plugin or reharvesting any records. You can simply replace the OaipmhHarvester plugin directory with the updated one.
You can download the OAI-PMH harvester version 1.0 at here.