The commit is:
https://codereview.qt-project.org/c/qt/qtbase/+/627786
It replaces some if conditions using "QThread::isMainThread()". But under corner case, this causes crash.

Minimal reproducer:

#include <QApplication>
#include <QThread>

int main(int argc, char** argv)
{
    std::thread([&]() {
        QApplication app(argc, argv);
    }).join();

    QApplication app(argc, argv);
}

The first QApplication in another thread is totally fine. But after that, during the initialization of the second QApplication, QThread gets confused:

!QThread::isMainThread() 

is somehow considered true, and if statement reaches

        && !QGuiApplicationPrivate::platformIntegration()->hasCapability(QPlatformIntegration::ThreadedPixmaps)) {

And it crashes there due to "platformIntegration()" returning nullptr;