This $Id should've stayed in there..

plugins/UsersManager/Controller.php

@@ -29,9 +29,12 @@ class Piwik_UsersManager_Controller extends Piwik_Controller_Admin
 
 		$view = Piwik_View::factory('UsersManager');
 		
+		$IdSitesAdmin = Piwik_SitesManager_API::getInstance()->getSitesIdWithAdminAccess();
 		$idSiteSelected = 1;
-		
+
+		if(count($IdSitesAdmin) > 0)
 		{
+			$defaultWebsiteId = $IdSitesAdmin[0];
 			$idSiteSelected = Piwik_Common::getRequestVar('idSite', $defaultWebsiteId);
 		}