xml example cannot be generated for response type of array

Q&A (please complete the following information)

• OS: Windows 10
• Browser: Firefox, Chrome, Edge
• Method of installation: npm
• Swagger-UI version: 3.11.0
• Swagger/OpenAPI version: Swagger 2.0

Content & configuration

I've reproduced this locally, but even easier to demonstrate, petstore has the same issue.

Example Swagger/OpenAPI definition:
https://generator.swagger.io/api/swagger.json

snippet from swagger.json

      responses:
        200:
          description: "successful operation"
          schema:
            type: "array"
            items:
              $ref: "#/definitions/Pet"

Describe the bug you're encountering

In swagger ui, when you have a GET that has a response that is a list, and you selected content type of xml, the Example Value has an error "XML example cannot be generated".

To reproduce...

Steps to reproduce the behavior:

1. Go to https://editor.swagger.io
2. Expand /pet/findByStatus
3. Make sure Response content type is application/xml
4. Example Value for status code 200 has "XML example cannot be generated"

Expected behavior

I would expect to see the example xml for the response.

Screenshots

Reproduce the problem in Swagger UI using the Swagger 2.0 definition at generator.swagger.io/api/swagger.json, then expand /pet/findByStatus in editor.swagger.io with application/xml selected. Trace the example generation path for the array response and compare it with the expected XML example; done means the Example Value no longer shows "XML example cannot be generated".