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