Incidence Dialog: don't show an empty message widget initially.
This is a regression introduced by commit 6b8797144d3b68a90aff.
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
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D20560