fix: init InvenioManager.doi as Null

dbrepo_ismir/invenio.py

@@ -129,6 +129,7 @@ class InvenioManager():
     def __init__(self, token: str, record_id: str|None = None):
         self.connector = InvenioConnector(token)
         self.record_id = record_id
+        self.doi = None
         
     def _validate_record_not_assigned(self):
         assert not self.record_id, "Record must not already be assigned!"