Disable 'Mirror Displays' on macOS causes all running Qt applications to crash

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.