NPE for dropdown lists

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36651 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#4578 - fix

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36652 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Discovered that the exception has something to do with synchronization, but maybe this is not the best way of solving it - this way a freeze might come in instead of the exception (to be continued tomorrow)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36653 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e