Circular reference in schema leads to Could not resolve reference: undefined undefined

Q&A (please complete the following information)

• OS: Alpine Linux
• Browser: Vivaldi
• Version: 2.9
• Method of installation: docker
• Swagger-UI version: actually unknown, should be latest
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

Example Swagger/OpenAPI definition:

openapi: "3.0.0"
info:
  title: "Test API For Self Reference"
  version: "1.0"


paths:
  /greeting:
    get:
      tags:
        - Client
      description: 'Get the current client session identifier.'
      responses:
        200: 
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Person'
        
components:
  schemas:
    Person:
      type: object
      required:
        - name
        - age
      properties:
        name:
         type: string
         example: 'John Doe'
        age:
          type: integer
          example: 42
        parent:
          $ref: '#/components/schemas/Person'
        brother:
          $ref: '#/components/schemas/Person'

Swagger-UI configuration options:
Where do I find it?

Describe the bug you're encountering

When expanding the Person schema, Swagger-UI displays this error on top of the page:
Resolver error at components.schemas.Person.properties.parent.$ref
Could not resolve reference: undefined undefined

To reproduce...

Steps to reproduce the behavior:

1. Scroll down to 'schemas'
2. Click on 'Person' to expand it
3. Scroll up to the top of the page
4. See error

Expected behavior

No error should occur.

No source file or test is named. Start by loading the provided OpenAPI 3.0 definition in Swagger UI and expanding the Person schema, then trace the schema resolver entry point used for recursive $ref values. Done means circular parent and brother references render without the “Could not resolve reference” error.