[BUG] NPE encountered when validating valid YAML

Description

Validation fails because of a nullpointerexception thrown deep inside of the parsing code with no helpful output on what (if anything) is wrong with the API spec.

Bug output:

$ openapi-generator validate -i api.yaml 
Validating spec (api.yaml)
[main] ERROR i.s.parser.SwaggerCompatConverter - failed to read resource listing
com.fasterxml.jackson.core.JsonParseException: Unrecognized token 'components': was expecting ('true', 'false' or 'null')
 at [Source: (String)"components:
  schemas:
    Status:
      properties:
        message:
          type: string
        status:
          type: string
      required:
      - status
      type: object
    Word:
      properties:
        word:
          type: string
      required:
      - word
      type: object
    WordList:
      properties:
        words:
          items:
            type: string
          type: array
      required:
      - words
      type: object
info:
  title: Sample API
  version: 0.0.1
op"[truncated 964 chars]; line: 1, column: 11]
	at com.fasterxml.jackson.core.JsonParser._constructError(JsonParser.java:1804)
	at com.fasterxml.jackson.core.base.ParserMinimalBase._reportError(ParserMinimalBase.java:673)
	at com.fasterxml.jackson.core.json.ReaderBasedJsonParser._reportInvalidToken(ReaderBasedJsonParser.java:2835)
	at com.fasterxml.jackson.core.json.ReaderBasedJsonParser._handleOddValue(ReaderBasedJsonParser.java:1889)
	at com.fasterxml.jackson.core.json.ReaderBasedJsonParser.nextToken(ReaderBasedJsonParser.java:747)
	at com.fasterxml.jackson.databind.ObjectMapper._readTreeAndClose(ObjectMapper.java:4042)
	at com.fasterxml.jackson.databind.ObjectMapper.readTree(ObjectMapper.java:2551)
	at io.swagger.parser.SwaggerCompatConverter.readResourceListing(SwaggerCompatConverter.java:210)
	at io.swagger.parser.SwaggerCompatConverter.read(SwaggerCompatConverter.java:123)
	at io.swagger.parser.SwaggerCompatConverter.readWithInfo(SwaggerCompatConverter.java:94)
	at io.swagger.parser.SwaggerParser.readWithInfo(SwaggerParser.java:42)
	at io.swagger.v3.parser.converter.SwaggerConverter.readLocation(SwaggerConverter.java:96)
	at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:16)
	at org.openapitools.codegen.cmd.Validate.run(Validate.java:46)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)
[main] ERROR i.s.parser.SwaggerCompatConverter - failed to read resource listing
com.fasterxml.jackson.core.JsonParseException: Unrecognized token 'components': was expecting ('true', 'false' or 'null')
 at [Source: (String)"components:
  schemas:
    Status:
      properties:
        message:
          type: string
        status:
          type: string
      required:
      - status
      type: object
    Word:
      properties:
        word:
          type: string
      required:
      - word
      type: object
    WordList:
      properties:
        words:
          items:
            type: string
          type: array
      required:
      - words
      type: object
info:
  title: Sample API
  version: 0.0.1
op"[truncated 964 chars]; line: 1, column: 11]
	at com.fasterxml.jackson.core.JsonParser._constructError(JsonParser.java:1804)
	at com.fasterxml.jackson.core.base.ParserMinimalBase._reportError(ParserMinimalBase.java:673)
	at com.fasterxml.jackson.core.json.ReaderBasedJsonParser._reportInvalidToken(ReaderBasedJsonParser.java:2835)
	at com.fasterxml.jackson.core.json.ReaderBasedJsonParser._handleOddValue(ReaderBasedJsonParser.java:1889)
	at com.fasterxml.jackson.core.json.ReaderBasedJsonParser.nextToken(ReaderBasedJsonParser.java:747)
	at com.fasterxml.jackson.databind.ObjectMapper._readTreeAndClose(ObjectMapper.java:4042)
	at com.fasterxml.jackson.databind.ObjectMapper.readTree(ObjectMapper.java:2551)
	at io.swagger.parser.SwaggerCompatConverter.readResourceListing(SwaggerCompatConverter.java:210)
	at io.swagger.parser.SwaggerCompatConverter.read(SwaggerCompatConverter.java:123)
	at io.swagger.parser.SwaggerCompatConverter.readWithInfo(SwaggerCompatConverter.java:94)
	at io.swagger.parser.SwaggerParser.readWithInfo(SwaggerParser.java:42)
	at io.swagger.v3.parser.converter.SwaggerConverter.readLocation(SwaggerConverter.java:96)
	at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:16)
	at org.openapitools.codegen.cmd.Validate.run(Validate.java:46)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)
Exception in thread "main" java.lang.NullPointerException
	at java.util.HashSet.<init>(HashSet.java:119)
	at org.openapitools.codegen.cmd.Validate.run(Validate.java:48)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)

openapi-generator version

4.1.3

OpenAPI declaration file content or url

components:
  schemas:
    Status:
      properties:
        message:
          type: string
        status:
          type: string
      required:
      - status
      type: object
    Word:
      properties:
        word:
          type: string
      required:
      - word
      type: object
    WordList:
      properties:
        words:
          items:
            type: string
          type: array
      required:
      - words
      type: object
info:
  title: Sample API
  version: 0.0.1
openapi: 3.0.2
paths:
  /word/{word}:
    get:
      description: Get a word
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Word'
  /word:
    post:
      description: Post a new word
      requestBody:
        content:
          schema: WordSchema
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Word'
    get:
      description: Get some words
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/WordList'
  /ping:
    get:
      description: Ping the API
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Status'
  /:
    get:
      description: Return discovery document

Command line used for generation

openapi-generator validate -i api.yaml

$ openapi-generator version
4.1.3

Steps to reproduce

1. Just run the validate command on the provided API spec

Start by reproducing the issue with the supplied API declaration and openapi-generator validate -i api.yaml on version 4.1.3. Trace the reported path through org.openapitools.codegen.cmd.Validate.run and io.swagger.parser.SwaggerCompatConverter; done means validation reports a useful parsing or specification error instead of throwing a NullPointerException.