The value can not actually be null at that point, but it's too
hard to spot for tools like coverity:
- before setting view, we chain up to the parent's acceptDrop()
- that calls _canAccept(), which we override and check for the view
Still, and extra ? doesn't hurt, and hopefully will make the tooling
happy.
CID 351269
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1950>
5b3036a6bf