Incidence Dialog: don't show an empty message widget initially.
Summary: This is a regression introduced by commit 6b879714. Looks like we can't trust Qt Designer to keep hidden things hidden, so better do it in actual code. Test Plan: "New Event" no longer shows a blue rectangle at the top of the dialog. Reviewers: kfunk, mlaurent Reviewed By: mlaurent Subscribers: kde-pim Tags: #kde_pim Differential Revision: https://phabricator.kde.org/D20560