QmlDesigner.PropertyEditor: Fix GeometrySection

This fixes a regression introduced by
commit 1da0d018c2dde2de048e80d1335875d6ce96c49c.

Task-number: QTCREATORBUG-12501
Change-Id: I96dd1c19afa6b6b0a56ef5f6ea5e01272aa1b774
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>