The custom field database model has changed in Netbox 2.7.
Therefore the initializer code, that was used before, broke.
As a user, you will need to update your custom_fields.yml file as
follows:

- type must be lowercase
  - the `selection` type was changed to `select`
- the filter_logic must be lower case

This is to achieve compatibility with the naming schema that
Netbox uses internally. It allows us to become forward compatible
in case Netbox ever introduces a new type for custom fields.

See the diff of this commit for further information how this is meant.