QXcbConnection::updateScreens accesses QGuiApplicationPrivate::platform_integration before it is constructed:

QGuiApplication::init_platform (qguiapplication.cpp:1008):
QGuiApplicationPrivate::platform_integration = QPlatformIntegrationFactory::create(name, arguments, argc, argv, platformPluginPath);

QXcbIntegrationPlugin::create (qxcbmain.cpp:50):
return new QXcbIntegration(parameters, argc, argv);

QXcbIntegration::QXcbIntegration (qxcbintegration.cpp:166):
m_connections << new QXcbConnection(m_nativeInterface.data(), m_canGrab, displayName);

QXcbConnection::QXcbConnection (qxcbconnection.cpp:333):
updateScreens();

QXcbConnection::updateScreens (qxcbconnection.cpp:243):
QXcbIntegration *integration = static_cast<QXcbIntegration *>(QGuiApplicationPrivate::platformIntegration());