Q_APPLICATION_STATIC needs to say that it's in #include <QtCore/QApplicationStatic>

According to documentation https://doc.qt.io/qt-6/qglobalstatic.html,
it should be enough to add #include <QGlobalStatic> in order to use Q_APPLICATION_STATIC macro.

But this is actually not true and you have to add #include <qapplicationstatic.h> into your code
to be able use it.