[Kickoff] Fix context menu not appearing on right-click

Summary: Right-clicking on list items to bring up a context menu broke with 48b32259e73f374a456ed836ba44e3effe2baee5. This fixes it. Thanks to @broulik for finding this; I'm submitting it on his behalf because he's super busy right now.

Test Plan: Right-click on a list item in Kickoff -> now a context menu appears

Reviewers: Plasma, hein, broulik, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16697