The recently introduced `activity` view (see rev. odoo/odoo@1de9496 has not
received all the consideration it deserves as a real view type (no RNG, not in
the valid view `type`, no doc, etc.). It was currently only working based on a
`_get_default_activity_view` method.
Even if the RNG is currently really basic, its grammar could evolve in the future.
This was causing an issue when opening Studio with this view as a default view
with an invalid type was created.