ocrd_validators.page_validator: cope with empty RO
@@ -182,6 +182,7 @@ def page_get_reading_order(ro, rogroup):
and an object ``rogroup`` with additional ReadingOrder element objects,
add all references to the dict, traversing the group recursively.
"""
+ regionrefs = list()
if isinstance(rogroup, (OrderedGroupType, OrderedGroupIndexedType)):
regionrefs = (rogroup.get_RegionRefIndexed() +
rogroup.get_OrderedGroupIndexed() +
