Don't crash with Qt 5.11+

Summary:
Based on the patch from https://bugs.kde.org/show_bug.cgi?id=371511
See also https://bugreports.qt.io/browse/QTBUG-72260

BUG: 371511

Reviewers: PHID-PROJ-odxxyyfgujhgbu6ergrt, mlaurent

Reviewed By: PHID-PROJ-odxxyyfgujhgbu6ergrt, mlaurent

Subscribers: marten, asturmlechner, kfunk, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D17563