Fixed bug in _getAllVariables causing failure when parent.top is None (e.g. when using multiple unions and queries are nested more than one level(?))
Fixed bug in _getAllVariables causing failure when parent.top is None (e.g. when using multiple unions and queries are nested more than one level(?))

armandobs14/rdflib,ssssam/rdflib,marma/rdflib,avorio/rdflib,dbs/rdflib,RDFLib/rdflib,armandobs14/rdflib,yingerj/rdflib,avorio/rdflib,marma/rdflib,RDFLib/rdflib,armandobs14/rdflib,dbs/rdflib,ssssam/rdflib,marma/rdflib,armandobs14/rdflib,RDFLib/rdflib,avorio/rdflib,yingerj/rdflib,dbs/rdflib,yingerj/rdflib,dbs/rdflib,RDFLib/rdflib,yingerj/rdflib,ssssam/rdflib,marma/rdflib,ssssam/rdflib,avorio/rdflib