Fix invalid pointer in script collection after opening popup

The script collection has a pointer to webcontents adapter, and when a
new adapter is adopted, the pointer in the script collection needs to
be updated.

Task-number: QTBUG-54419
Change-Id: Ia054e1281ab4db637beab570abda752074dc9280
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>