A resize event delivered after closing the platform window
was causing the stored frame margins to be cleared.
Bail out of QWidgetWindow::updateMargins() if the
platform window is null.
Pick-to: 5.15
Fixes: QTBUG-79147
Change-Id: Iebbc90c3cccafa209cd720baedf45affb3f3c2b8
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>