cannot activate file system watcher in QFileSystemModel

void QFileInfoGatherer::setWatching(bool v)
{
#if QT_CONFIG(filesystemwatcher)
    QMutexLocker locker(&mutex);
    if (v != m_watching) {
        if (!v) {
            delete m_watcher;
            m_watcher = nullptr;
        }
        m_watching = v;
    }
#else
    Q_UNUSED(v);
#endif
}