Fix crash when loaded in Kontact

Summary:
When loaded via the part, the PageView has the splitter as parent and so
is deleted as soon as the part gets deleted. It means it ends up deleted
before ApplicationComponents in that case, leading to a call to setModel
on an already deleted PageView.

Reviewers: franckarrecot, bensi, dfaure, jriddell

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D6284