qt: fix crash at exit on Windows 8/10/11 and crash at startup on Windows 7 regressions

When ::destroyMainInterface() already gets called before, ::unloadGUI() gets called again as well and with the recent introduction of m_quickView->setSource(QUrl()); there without first checking for null pointer it causes null pointer dereference as ::destroyMainInterface() already destroys the ::m_quickView instance.

At the same time, calling ::destroyMainInterface() during destruction causes the same null pointer dereference, since if initialization fails there is no ::m_quickView instance. This is a case with Windows 7 where direct composition is not available, which causes crash at start.

Regression since !6503 (merged).

Request review @chub.