Add runtime guard that pages are KCMs in KCMultiDialog
Summary:
This situation can come up if a KCMultiDialog subclass adds pages
directly that aren't KCModules.
BUG: 405440
Test Plan: acheronuk tested this patch on a crashing example
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D19888