NPE for Mirror

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@34967 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Ticket #3874: improving my solution commited earlier

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@34968 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e