Comment 1 for bug 1455122

Confirmed. This does not happen when upgrading from 1.9 -> 1.10. It appears that it's due to the blocktype category code that we added in 15.04 to store block sortorders.

While we don't yet support fully uninstalling plugins (that's bug 505732), Mahara shouldn't crash like this if you've just deleted the plugin.

Another possible workaround to those listed above, would be to put in a "dummy" placeholder plugin with no functionality, but with the same name as the expected plugin. But I'm going to try to get this bug fixed for 15.04.1.

Cheers,
Aaron