Cannot use plain/text as response type

[JKostov]

Describe the bug
When using plain/text as a response type there is an error: TypeError: Cannot use 'in' operator to search for '$ref' in undefined

To Reproduce

Method getResponseSchemaAndNames for the following specification doesn't work properly.

openapi: 3.0.3
info:
  version: v1
  title: Plain text example
paths:
  "/test":
    get:
      summary: Get string value
      responses:
        "200":
          description: Returns value as a string
          content:
            text/plain:
              example: example

Expected behavior
It should return string as a response type.

[Alan-Cha]

@JKostov Yes, this does indeed seem to be an issue. Thank you for reporting this issue!