How to read screen information from QGuiApplication to put in qputenv environment variable?

Question

Aim is to run this application on screens with different PPI.

Qt 5.12

The problem is that qputenv does not work if written after the initialization of QGuiApplication.

I want to calculate scale according to the information from QGuiApplication.

How do I do it? The following code throws a segmentation fault.

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QScreen>
#include <QDebug>
int main(int argc, char *argv[])
{
    qreal abc = QGuiApplication::primaryScreen()->physicalDotsPerInch();

    qDebug() << "null " << abc;

    qputenv("QT_SCALE_FACTOR", "2");

    QGuiApplication app(argc, argv);
    
    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}