RequestBody with 'x-www-urlencoded' render error

[wsw0108]

Q&A (please complete the following information)

• OS: Windows 10
• Browser: chrome
• Version: 89
• Method of installation: dist assets
• Swagger-UI version: 3.47.1
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

Example Swagger/OpenAPI definition:

openapi: 3.0.0
info:
  title: "test"
  version: 1.0.0
paths:
  /test:
    post:
      requestBody:
        description: "test"
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Message'
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                data:
                  $ref: '#/components/schemas/Message'
            encoding:
              data:
                contentType: application/json
      responses:
        200:
          description: "OK"
          content:
            application/json:
              schema:
                type: object
components:
  schemas:
    Message:
      type: object
      properties:
        text:
          type: string

Swagger-UI configuration options:
using https://editor.swagger.io/

SwaggerUI({
  // your config options here
})

Describe the bug you're encountering

To reproduce...

Steps to reproduce the behavior:

1. Click "Try it out" with content type 'application/x-www-form-urlencoded'
2. See error

Expected behavior

The edit textarea should fill with default value when select content type 'application/json'.

Screenshots

Additional context or thoughts

[hkosova]

Duplicate of #7268.