[HTML] OAS3.0 code without components block throws 500

Description

If your OAS3.0 YAML definition doesn't have a components block, the editor doesn't show any errors - but clicking generate client throws an HTTP 500 error in the Chrome Dev Console.

Swagger-codegen version

Whatever is running at generator3.swagger.io/v2/generate.json

Swagger declaration file content or url

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Swagger Petstore
  license:
    name: MIT
servers:
  - url: http://petstore.swagger.io/v1
paths:
  /pets:
    get:
      summary: List all pets
      operationId: listPets
      tags:
        - pets
      parameters:
        - name: limit
          in: query
          description: How many items to return at one time (max 100)
          required: false
          schema:
            type: integer
            format: int32
      responses:
        '200':
          description: A paged array of pets
          headers:
            x-next:
              description: A link to the next page of responses
              schema:
                type: string
          content:
            application/json:    
              schema:
               properties:
                id:
                  type: integer
                  format: int64
                name:
                  type: string
                tag:
                  type: string
        default:
          description: unexpected error
          content:
            application/json:
              schema:
               properties:
                id:
                  type: integer
                  format: int64
                name:
                  type: string
                tag:
                  type: string
    post:
      summary: Create a pet
      operationId: createPets
      tags:
        - pets
      responses:
        '201':
          description: Null response
        default:
          description: unexpected error
          content:
            application/json:
              schema:
               properties:
                id:
                  type: integer
                  format: int64
                name:
                  type: string
                tag:
                  type: string
  /pets/{petId}:
    get:
      summary: Info for a specific pet
      operationId: showPetById
      tags:
        - pets
      parameters:
        - name: petId
          in: path
          required: true
          description: The id of the pet to retrieve
          schema:
            type: string
      responses:
        '200':
          description: Expected response to a valid request
          content:
            application/json:
              schema:
               properties:
                id:
                  type: integer
                  format: int64
                name:
                  type: string
                tag:
                  type: string
        default:
          description: unexpected error
          content:
            application/json:
              schema:
               properties:
                id:
                  type: integer
                  format: int64
                name:
                  type: string
                tag:
                  type: string
# components:
#   schemas:
#     Pet:
#       required:
#         - id
#         - name
#       properties:
#         id:
#           type: integer
#           format: int64
#         name:
#           type: string
#         tag:
#           type: string
#     Pets:
#       type: array
#       items:
#         $ref: "#/components/schemas/Pet"
#     Error:
#       required:
#         - code
#         - message
#       properties:
#         code:
#           type: integer
#           format: int32
#         message:
#           type: string

Command line used for generation

n/a

Steps to reproduce

1. Paste above YAML in editor.swagger.io (use HTTP because of this: https://github.com/swagger-api/swagger-editor/issues/1762)
2. Click generate client
3. Observe 500 error in Chrome Dev Console

Note that the editor shows no errors with components commented out. If you un-comment the components section, it works.

Suggest a fix/enhancement

If the editor doesn't show an error, the generator should work. I'm not sure what is going wrong here to cause this problem.