Problem with Callbacks having extensions

As per OpenAPI (Version: 3.0.3), Callback object (https://swagger.io/specification/#callback-object) May be extended with Specification Extensions (https://swagger.io/specification/#specification-extensions).

However the swagger-parser (2.0.17) fails to parse the following and throws “com.fasterxml.jackson.databind.node.TextNode cannot be cast to com.fasterxml.jackson.databind.node.ObjectNode”

{
  "openapi": "3.0.0",
  "info": {
    "version": "0.0.0",
    "title": "test"
  },
  "paths": {
    "/subscribe": {
      "post": {
        "summary": "Subscribe to a webhook",
        "requestBody": {
          "required": true,
          "content": {
            "application/json": {
              "schema": {
                "type": "object",
                "properties": {
                  "callbackUrl": {
                    "type": "string",
                    "format": "uri",
                    "example": "https://myserver.com/send/callback/here"
                  }
                },
                "required": [
                  "callbackUrl"
                ]
              }
            }
          }
        },
        "responses": {
          "201": {
            "description": "Webhook created"
          }
        },
        "callbacks": {
          "myEvent": {
            "x-amazon-apigateway-authtype": "oauth2",
            "{$request.body#/callbackUrl}": {
              "post": {
                "requestBody": {
                  "required": true,
                  "content": {
                    "application/json": {
                      "schema": {
                        "type": "object",
                        "properties": {
                          "message": {
                            "type": "string",
                            "example": "Some event happened"
                          }
                        },
                        "required": [
                          "message"
                        ]
                      }
                    }
                  }
                },
                "responses": {
                  "200": {
                    "description": "Your server returns this code if it accepts the callback"
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Start by reproducing the failure with the OpenAPI JSON included in the issue, then trace callback deserialization and how callback extensions are handled. Done means the provided document parses successfully without the TextNode/ObjectNode cast error and the callback extension remains supported.