import custom fields data bug

ERROR 2021-04-26 10:04:02,320 importer 3140 140325667661632 Problem with importing study subject: cov-895455,cov-895455
Traceback (most recent call last):
  File "/var/www/scheduling-system/smash/web/importer/importer.py", line 43, in execute
    self.import_study_subject(study_subject)
  File "/var/www/scheduling-system/smash/web/importer/importer.py", line 85, in import_study_subject
    study_subject.set_custom_data_value(custom_field_value.study_subject_field, custom_field_value.value)
  File "/var/www/scheduling-system/smash/web/models/study_subject.py", line 249, in set_custom_data_value
    study_subject_field=custom_study_subject_field))
  File "/var/www/scheduling-system/env/lib/python3.6/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/var/www/scheduling-system/env/lib/python3.6/site-packages/django/db/models/query.py", line 447, in create
    obj.save(force_insert=True, using=self.db)
  File "/var/www/scheduling-system/env/lib/python3.6/site-packages/django/db/models/base.py", line 702, in save
    "unsaved related object '%s'." % field.name
ValueError: save() prohibited to prevent data loss due to unsaved related object 'study_subject'.