monitoring: Fix exception when scheduling a downtime for multiple hosts

Introduced w/ the Icinga 2 related change for disabling child options.

fixes #10902

modules/monitoring/application/controllers/HostsController.php

@@ -193,6 +193,7 @@ class HostsController extends Controller
         $this->assertPermission('monitoring/command/downtime/schedule');
 
         $form = new ScheduleHostDowntimeCommandForm();
+        $form->setBackend($this->backend);
         $form->setTitle($this->translate('Schedule Host Downtimes'));
         $this->handleCommandForm($form);
     }