Fix creating itip import via mail when falling back to the default calendar.

plugins/calendar/calendar.php

@@ -2922,8 +2922,9 @@ class calendar extends rcube_plugin
       if (!$calendar && !$dontsave)
          $calendar = $this->get_default_calendar(true, $event['sensitivity'] == 'confidential');
 
-      if(!$driver)
-        $driver = $this->get_driver_by_cal($calendar);
+      if(!$driver) {
+        $driver = $this->get_driver_by_cal($calendar["id"]);
+      }
 
       $metadata = array(
         'uid' => $event['uid'],