Fix crash when using openIn on newly created view

When a link triggers a NewViewRequest, and the latter creates
a new window with a new WebEngineView inside, the "adapter" pointer
can still be NULL when the adoptWebContents is called by openIn.
Therefore is it necessary to test the adapter pointer before using it.

Task-number: QTBUG-55765
Change-Id: Ia1a299dd65c229705462c9444b81d7b26567fc09
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>