NPE fix for CAS (SingularWS related)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31781 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Fix for #4087

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31782 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e