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.

(cherry picked from commit bfc2683a

)
Task-number: QTBUG-55765
Change-Id: Iaa7cb4e8c7780a2e3f1a8c85b7b5da0ec541b2f3
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>