QmlDesigner.PropertyEditor: Fixing crash

The designer was crashing when trying to anchor the root item.
The root item cannot be anchored.

Task-number: QTCREATORBUG-11078
Change-Id: I1aec1ad5b15de8c2baf3b76358d02347fb3a124c
Reviewed-by: Marco Bubke <marco.bubke@digia.com>