Workaround QTBUG-47066 to fix crash on startup

Summary:
The lambda returns a QStringBuilder which contains dangling references.
Work around that by converting to QString inside the lambda's context.

Test Plan: Does not crash anymore.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15498