avoid exception

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36376 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
preview for SVG images #4530

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36377 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e