Fix a warning message

Summary:
Fix warning messages such as : file:///home/$USER/kde/usr/share/plasma/plasmoids/org.kde.plasma.kickoff/contents/ui/KickoffItem.qml:142:9: Unable to assign [undefined] to QString
Occurring when hovering items in the document history section of kickoff.

Test Plan: No more warning messages in ~/.xsessions-error

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22090