I can confirm I'm getting this error as well.
Although it doesn't seem to be breaking Piwik for me in any way
[Sat Dec 21 17:27:09 2013] [error] [client ***.***.***.***] PHP Fatal error: Call to a member function rewriteAssetsPathToTheme() on a non-object in /srv/vhosts/blach/docroot/core/AssetManager/UIAssetMerger.php on line 181, referer: https://examle.com/index.php?module=CoreUpdater&action=oneClickResults
Although it doesn't seem to be breaking Piwik for me in any way