public function handle(GetGroupHeaderEvent $event)
    {
        if ((null !== $event->getValue()) || !$this->scopeDeterminator->currentScopeIsBackend()) {
            return;
        }

        $environment = $event->getEnvironment();
        $property    = $environment
            ->getDataDefinition()
            ->getPropertiesDefinition()
            ->getProperty($event->getGroupField());

        // No property? Get out!
        if (!$property) {
            $event->setValue('-');
            return;
        }

        $value = $this->formatGroupHeader(
            $environment,
            $event->getModel(),
            $property,
            $event->getGroupingMode(),
            $event->getGroupingLength()
        );

        if (null !== $value) {
            $event->setValue($value);
        }
    }