Make sure MessageViewerSettings::self() is called before the base class one

Summary:
This is a dirty workaround and fixes akonadiconsole crashing on startup.
A proper solution would need a larger review of the settings handling
here I guess, ideally getting rid of the base-class self() method to
prevent this from happening in the first place.

Reviewers: mlaurent, dfaure

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28588