Crash on macOS in QCocoaWindow::windowDidChangeScreen() when unplugging monitors

Summary:

An application compiled with Qt 5.12.1 occasionally crashes when unplugging the external monitors from a MacBookPro running on macOS 10.14.3.

Steps to reproduce:

1. I have a MacBookPro with a Thunderbolt Dock and 2 screens connected to the Dock.
2. Run an application compiled with Qt 5.12.1
3. Detach the Thunderbolt Dock from the MacBookPro

Result: The application sometimes crashes.

More information:

The crash occurs in QCocoaWindow::windowDidChangeScreen():

void QCocoaWindow::windowDidChangeScreen()
{
    if (!window())
        return;

    const bool wasRunningDisplayLink = static_cast<QCocoaScreen *>(screen())->isRunningDisplayLink();

Note the call to the function screen() which can return a nullptr:

/*!
    Returns the platform screen handle corresponding to this platform window,
    or null if the window is not associated with a screen.
*/
QPlatformScreen *QPlatformWindow::screen() const
{
    QScreen *scr = window()->screen();
    return scr ? scr->handle() : nullptr;
}

Suggested fix:

The QCocoaWindow::windowDidChangeScreen() function should check if screen() returns a nullptr pointer before using it. One possible fix would be to modify QCocoaWindow::windowDidChangeScreen() as follow:

void QCocoaWindow::windowDidChangeScreen()
{
    if (!window())
        return;

    if (!screen())
        return;

    const bool wasRunningDisplayLink = static_cast<QCocoaScreen *>(screen())->isRunningDisplayLink();