fixed possible NPE in XLSInstanceReader

Change-Id: Ia37907c1a65cbe0685e810daf20b1ddcc8a483ef
Reviewed-on: http://gerrit.esdi-community.eu/3136
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>