macOS: Check that platform window is valid after delivering close event

Fixes: QTBUG-78814
Pick-to: 5.15
Change-Id: I551024c5d777999f7c79d21fd7b7de061c18f971
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/cocoa/qnsview_mouse.mm

@@ -400,6 +400,8 @@
                 selfClosed = self == popup->view();
                 QWindowSystemInterface::handleCloseEvent(popup->window());
                 QWindowSystemInterface::flushWindowSystemEvents();
+                if (!m_platformWindow)
+                    return; // Bail out if window was destroyed
             }
             // Consume the mouse event when closing the popup, except for tool tips
             // were it's expected that the event is processed normally.