Cocoa: Prevent crash on focusObjectChanged()

Don’t crash on focusObjectChanged to a QWindow with
a null QPlatformWindow.

Change-Id: Ibf426521d6c8f8b2b2e94d5b92020fa510a74133
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoainputcontext.mm

@@ -129,6 +129,8 @@ void QCocoaInputContext::focusObjectChanged(QObject *focusObject)
             return;
 
         QCocoaWindow *window = static_cast<QCocoaWindow *>(mWindow->handle());
+        if (!window)
+            return;
         QNSView *view = qnsview_cast(window->view());
         if (!view)
             return;