Removing metadata type causes exception

I inserted a test metadata type. It was added to a document type and then I removed it. I got an internal server error and the removal failed.

This also halted the document processing.

It would be useful if I could manually repair this as there's already a considerable number of documents in the database (approx. 5k).

Error log is here:

mayan.apps.common.middleware.error_logging <21093> [ERROR] "process_exception() line 18 ^[[31;1mException caught by request middleware; <WSGIRequest: POST '/metadata/metadata_types/3/delete/'>, update or delete on table "metadata_metadatatype" violates foreign key constraint "metadata_documentmet_metadata_type_id_b3eee3ed_fk_metadata_" on table "metadata_documentmetadata"
DETAIL:  Key (id)=(3) is still referenced from table "metadata_documentmetadata".
^[[0m"
Traceback (most recent call last):
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/backends/base/base.py", line 236, in _commit
    return self.connection.commit()
psycopg2.errors.ForeignKeyViolation: update or delete on table "metadata_metadatatype" violates foreign key constraint "metadata_documentmet_metadata_type_id_b3eee3ed_fk_metadata_" on table "metadata_documentmetadata"
DETAIL:  Key (id)=(3) is still referenced from table "metadata_documentmetadata".


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/mayan/apps/common/mixins.py", line 487, in dispatch
    ).dispatch(request, *args, **kwargs)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/views/generic/edit.py", line 269, in post
    return self.delete(request, *args, **kwargs)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/mayan/apps/common/generics.py", line 595, in delete
    result = super(SingleObjectDeleteView, self).delete(request, *args, **kwargs)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/mayan/apps/common/mixins.py", line 55, in delete
    self.object.delete()
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/models/base.py", line 974, in delete
    return collector.delete()
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/models/deletion.py", line 308, in delete
    sender=model, instance=obj, using=self.using
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/transaction.py", line 223, in __exit__
    connection.commit()
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/backends/base/base.py", line 262, in commit
    self._commit()
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/backends/base/base.py", line 236, in _commit
    return self.connection.commit()
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/home/mayan/mayan-edms/lib/python3.7/site-packages/django/db/backends/base/base.py", line 236, in _commit
    return self.connection.commit()
django.db.utils.IntegrityError: update or delete on table "metadata_metadatatype" violates foreign key constraint "metadata_documentmet_metadata_type_id_b3eee3ed_fk_metadata_" on table "metadata_documentmetadata"
DETAIL:  Key (id)=(3) is still referenced from table "metadata_documentmetadata".