Fix error when there is an array with null value for treatment
Fix error when there is an array with null value for treatment

HarvardEconCS/turkserver-meteor,VirtualLab/turkserver-meteor,VirtualLab/turkserver-meteor,HarvardEconCS/turkserver-meteor,HarvardEconCS/turkserver-meteor,TurkServer/turkserver-meteor,VirtualLab/turkserver-meteor,TurkServer/turkserver-meteor,TurkServer/turkserver-meteor