Fix error on directory cache when hot reloading Studio jar

Caused by: java.lang.NullPointerException
	at org.nuxeo.ecm.directory.DirectoryCacheFlusher.handleEvent(DirectoryCacheFlusher.java:59)
	at org.nuxeo.runtime.services.event.EventService.sendEvent(EventService.java:141)
	at org.nuxeo.runtime.services.event.EventService.sendEvent(EventService.java:108)
	at org.nuxeo.runtime.reload.ReloadComponent.flush(ReloadComponent.java:120)
	at org.nuxeo.connect.update.task.live.commands.Flush.flush(Flush.java:50)
	at org.nuxeo.connect.update.task.live.LiveUninstallTask.flush(LiveUninstallTask.java:46)
	at org.nuxeo.connect.update.task.standalone.CommandsTask.doRun(CommandsTask.java:129)
	at org.nuxeo.connect.update.task.standalone.UninstallTask.doRun(UninstallTask.java:59)
	at org.nuxeo.connect.update.task.live.LiveUninstallTask.doRun(LiveUninstallTask.java:39)
	at org.nuxeo.connect.update.task.standalone.AbstractTask.run(AbstractTask.java:247)
	at org.nuxeo.connect.client.jsf.AppCenterViewsManager$StudioAutoInstaller.performTask(AppCe