Re: unknown error

For a good intro into writing a plugin read: http://geekmonkey.org/articles/34-how-to-write-a-piwik-plugin

You receive these notices because you need to call setGeneralVariablesView() on the View object. The Multisites plugin does that in: https://github.com/piwik/piwik/blob/master/plugins/MultiSites/Controller.php#L169