Properly get calendar and driver instances if requested.

plugins/calendar/calendar.php

@@ -2914,11 +2914,12 @@ class calendar extends rcube_plugin
       $dontsave = ($_REQUEST['_folder'] === '' && $event['_method'] == 'REQUEST');
 
       // select default calendar except user explicitly selected 'none'
-      if (!$calendar && !$dontsave)
-         $calendar = $this->get_default_calendar(true, $event['sensitivity'] == 'confidential');
-
-      if(!$driver)
-        $driver = $this->get_driver_by_cal($calendar);
+      if(!$dontsave) {
+        if (!$calendar) {
+          $calendar = $this->get_default_calendar(true, $event['sensitivity'] == 'confidential');
+          $driver = $this->get_driver_by_cal($calendar["id"]);
+        }
+      }
 
       $metadata = array(
         'uid' => $event['uid'],