I've noticed a problem with newly registered sites and the MultiSitesPlugin. As soon as you register a site via the API, then go straight to the Multi Site widget, you get an error like this:

Notice: Undefined property: Piwik_DataTable_Simple::$rows in /home/stats/piwik/core/DataTable.php on line 457 Backtrace -->
#0 Piwik_ErrorHandler(8, Undefined property: Piwik_DataTable_Simple::$rows, /home/stats/piwik/core/DataTable.php, 457, Array ()) called at [/home/stats/piwik/core/DataTable.php:457]
#1 Piwik_DataTable->rebuildIndex() called at [/home/stats/piwik/core/DataTable.php:433]
#2 Piwik_DataTable->getRowFromLabel(0) called at [/home/stats/piwik/core/DataTable/Simple.php:54]
#3 Piwik_DataTable_Simple->getColumn(0) called at [/home/stats/piwik/plugins/MultiSitesPlugin/Controller.php:156]
#4 Piwik_MultiSitesPlugin_Controller->getSummary(Piwik_DataTable_Array Object ([=> Array (), => Array ([=> Piwik_DataTable_Simple Object ( => Array ([=> Piwik_DataTable_Row Object (c => Array ([=> Array (label

An example of the API call I'm using is:

http://domain.com/piwik/?module=API&method=SitesManager.addSite&siteName=example.org&urls[0]=http://example.org&token_auth=XXXXXXXXXXXXX