macOS: Bail out early when handling shortcut event results in closing window

When Cmd+W is used to close a window, check if m_platformWindow is still valid
before accessing window property.

Task-number: QTBUG-63389
Change-Id: I9abda19b8482e7a1fd07b07d8981b6a768e96c2e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qnsview.mm

@@ -1559,6 +1559,10 @@ static QTabletEvent::TabletDevice wacomTabletDevice(NSEvent *theEvent)
         if (m_composingText.isEmpty()) {
             m_sendKeyEvent = !QWindowSystemInterface::handleShortcutEvent(window, timestamp, keyCode,
                 modifiers, nativeScanCode, nativeVirtualKey, nativeModifiers, text, [nsevent isARepeat], 1);
+
+            // Handling a shortcut may result in closing the window
+            if (!m_platformWindow)
+                return true;
         }
 
         QObject *fo = m_platformWindow->window()->focusObject();