Enabling devel breaks the entity.entity_importer_field_mapping.collection route

Problem/Motivation

Enabling devel breaks the entity.entity_importer_field_mapping.collection route

Steps to reproduce

- Enable entity_import
- Enable devel
- Create an entity_importer, for example: CSV, for content entities, named test_devel.
- Add a mapping
- Navigate to /admin/config/system/entity-importer/test_devel/field-mapping
- You get a WSOD

Proposed resolution

See attached patch.