I was hitting one more annoying crash with a backtrace inside QCache,
because we didn't set the ownership propetly for one last model. Now I
don't hit it anymore.
I'm 99% sure of the recent crashes we've been seeing are double-frees,
the QCache one me and Duha encountered must be one. The QCache is in
charge of the one in ContentProvider, so it will sometimes try to delete
or access something already destroyed by the QML engine.
While I'm at it, I also made sure to check every other Q_INVOKABLE to
ensure we don't hit this elsewhere.
CCBUG: 502747
