Hello @mmonty,
You just have to update the Sydney theme, new Sydney Toolbox plugin version won’t work with old Sydney theme version.
Kind Regards, Roman.
Sorry but why is this coded in a way it breaks the site? It should be prepared for outdated themes and put the message like the one you just posted.
So we know what to do.
It can’t be that we must kill the plugin via FTP to get the site back online.