adding nil check when adding accessibilityElement to GREYElementHierarchy (#91)
adding nil check when adding accessibilityElement to GREYElementHierarchy (#91)

adding nil check when adding accessibilityElement to subViewSet sometimes it can be nil.
sskhandp/EarlGrey,ed-chin/EarlGrey,sskhandp/EarlGrey,artyom-razinov/EarlGrey,saurabhj80/EarlGrey,saurabhj80/EarlGrey,ed-chin/EarlGrey,saurabhj80/EarlGrey,saurabhj80/EarlGrey,google/EarlGrey,sskhandp/EarlGrey,ed-chin/EarlGrey,saurabhj80/EarlGrey,google/EarlGrey,sskhandp/EarlGrey,ed-chin/EarlGrey,google/EarlGrey,artyom-razinov/EarlGrey,artyom-razinov/EarlGrey,artyom-razinov/EarlGrey,artyom-razinov/EarlGrey,ed-chin/EarlGrey,sskhandp/EarlGrey,google/EarlGrey,google/EarlGrey