Defensive code

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25850 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Prevent NPE in CAS test

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25851 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e