As Coverity (CID 22302) points out, qobject_cast<> can return NULL; if
we don't get a document layout, then we have no frame decorations to
add to it.

Change-Id: I2cd428456b3a3b24a28a5dd18ce948a8e95d21cb
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>