fix crash if notification is closed and there is no primary backend

src/libsnore/snore.cpp

@@ -177,8 +177,10 @@ bool SnoreCore::setPrimaryNotificationBackend(const QString &backend)
 void SnoreCore::requestCloseNotification(Notification n, Notification::CloseReasons r)
 {
     Q_D(SnoreCore);
+    if (d->m_notificationBackend) {
         d->m_notificationBackend->requestCloseNotification(n, r);
     }
+}
 
 bool SnoreCore::primaryBackendSupportsRichtext()
 {