Description

When you disable the 'Mirror Displays' checkbox on macOS, all running Qt applications are crashing. See attached video to see Qt Creator crashing.

Reproducibility

• This issue is reproducible with Qt 5.13.1 on macOS 10.15.1

Steps to reproduce

1. Attach an external display to a Mac (tested with iMac and MacBook Pro). I used a Thunderbolt Dock and Display Port to connect the external display.
2. Launch any Qt application on the external screen. For example Qt Creator.
3. Open System Preferences > Displays > Arrangement
4. Check "Mirror Displays" if it is not checked
5. Uncheck "Mirror Displays"

Result: All running Qt applications (for example Qt Creator) are crashing

Qt Support has tested and confirms the crash.

More info

 The crash occurs in QCocoaScreen::initializeScreens() in qcocoascreen.mm.

As you can see in this code, there is no check for NULL pointer for the variable mainDisplay and cocoaScreen:

            // Detect changes to the primary screen immediately, instead of
            // waiting for a display reconfigure with kCGDisplaySetMainFlag.
            // This ensures that any property updates to the other screens
            // will be in reference to the correct primary screen.
            QCocoaScreen *mainDisplay = QCocoaScreen::get(CGMainDisplayID());
            if (QGuiApplication::primaryScreen()->handle() != mainDisplay) {
                mainDisplay->updateProperties();
                qCInfo(lcQpaScreen) << "Primary screen changed to" << mainDisplay;
                QWindowSystemInterface::handlePrimaryScreenChanged(mainDisplay);
            }

            if (cocoaScreen == mainDisplay)
                return; // Already reconfigured

            cocoaScreen->updateProperties();
            qCInfo(lcQpaScreen) << "Reconfigured" << cocoaScreen;

If the variables mainDisplay or cocoaScreen are NULL - which is what seems to be the case when disabling "Mirror Displays" - the application is crashing.