fix crash on Properties block in Export item

The merged Export item was lacking a file context.
Added a QBS_CHECK in instantiateModule to ensure a valid file context
of the module instance scope.

Task-number: QBS-550
Change-Id: I04368ba2e290a9af70f22e9c4c07324e295da4f0
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>