QmlDesigner: Fix crash for master document handling

Happens for dragging a list view in the document.

Change-Id: Id8d00fc880789a0e5e85d455850bb1ec3eaac434
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Squish: Enhance tst_default_settings

Change-Id: I150d4d53a6e6e93749f9cdbfe33971633805573a
Reviewed-by: Robert Loehning <robert.loehning@digia.com>