I opened the eav_attribute_group table and saw that some records has null in attribute_group_code column:
SELECT * FROM `eav_attribute_group` WHERE attribute_set_id IN (
SELECT attribute_set_id FROM eav_attribute_set WHERE entity_type_id =4
)
This is because of the incorrect attribute group identifier generation algorithm:
Attribute group code generation algorithm is incorrect because it can return null for an attribute group with non-Latin name