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