Add guard for missing kirigami

Summary:
We rely on the component being created correctly later in the module,
better to fatal with a useful messgae than crash.

CCBUG: 405023

Test Plan: Regular KCM still loaded fine

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25115