QmlDesigner.ContextMenu: Crash fix

The root item cannot be "filled".
Adding a try catch block just in case.

Task-number: QTCREATORBUG-10476
Change-Id: Ic5c5efddd78194275d7d6e393eb945d6c9d89ed0
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>