Crash when using asynchronous Loaders

1. Run the demo app on Mac.
2. Click "File->Create QuickView"
3. In a newly created window click "Dock" on the first button
4. Then click “Undock” and immediately press Escape key.

Repeating steps 3 and 4 might be necessary several times as crash doesn't have 100% repro rate.

Call-stack after crash:
0 QtGui 0x00000001037265e8 QWindow::flags() const + 8
1 libqcocoa.dylib 0x0000000109015fd1 -[QNSView keyUp:] + 33
2 com.apple.AppKit 0x00007fff87748a02 -[NSWindow _reallySendEvent:] + 4507
3 com.apple.AppKit 0x00007fff871d116e -[NSWindow sendEvent:] + 446
4 libqcocoa.dylib 0x000000010900a177 -[QNSWindow superSendEvent:] + 39
5 libqcocoa.dylib 0x0000000109009b0b -[QNSWindowHelper handleWindowEvent:] + 411
6 com.apple.AppKit 0x00007fff871833d1 -[NSApplication sendEvent:] + 4055
7 libqcocoa.dylib 0x000000010902028e -[QNSApplication sendEvent:] + 78
8 com.apple.AppKit 0x00007fff8700f608 -[NSApplication run] + 711
9 libqcocoa.dylib 0x000000010901ca0d QCocoaEventDispatcher::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 2189
10 QtCore 0x000000010330b0ac QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 396
11 QtCore 0x000000010330e56c QCoreApplication::exec() + 364
12 QtWidgets 0x0000000103dbb9f9 QApplication::exec() + 9