Ensure the tupleHolder is a list of length 6
Ensure the tupleHolder is a list of length 6

This caused failures under Python 3.6 when exporting volumes.

mathturtle/tomviz,thewtex/tomviz,cjh1/tomviz,OpenChemistry/tomviz,cryos/tomviz,thewtex/tomviz,OpenChemistry/tomviz,cryos/tomviz,mathturtle/tomviz,cjh1/tomviz,cryos/tomviz,cjh1/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,thewtex/tomviz