QmlDesigner.NodeInstances: Change Assert in exit

Change-Id: I9a19ab772db406b0b1e6f9b76b997dcee17bccbf
Reviewed-on: http://codereview.qt-project.org/5244


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>