[BUG] validate crashes with a NullReferenceException for schema errors in another file

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When "outsourcing" your data structure into a seperate yaml file and referncing a schema from it, having unexpected attribute paths will lead to validate crashing with a null pointer exception.
Making the same error in one file (i.e. not using $ref produces a readable error)

Specifically, someone in my team misspelled properties as propertiers in a file with around 1.5k lines during a major update. This took a solid day to find...

openapi-generator version

$ java --version
openjdk 11.0.12 2021-07-20
OpenJDK Runtime Environment (build 11.0.12+7-post-Debian-2)
OpenJDK 64-Bit Server VM (build 11.0.12+7-post-Debian-2, mixed mode, sharing)


$ java -jar ./openapi-generator-cli.jar --version
openapi-generator-cli 5.2.1
  commit : 8403e59
  built  : 2021-08-16T12:52:51Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

api_standalone.yaml

openapi: 3.0.1
info:
  description: test api
  title: Test Api
  version: 1.0.0

paths:
  /hello:
    post:
      description: test function
      requestBody:
        description: Dummy API for generating Ecm Technique
        content:
          application/json:
            schema:
              description: A simple data structure
              propertiers:
                id:
                  description: a unique number
                  type: integer
                  format: int32
                name:
                  description: a name
                  type: string
      responses:
        default:
          description: no reply
          content: {}

api_with_ref.yaml

openapi: 3.0.1
info:
  description: test api
  title: Test Api
  version: 1.0.0

paths:
  /hello:
    post:
      description: test function
      requestBody:
        description: Dummy API for generating Ecm Technique
        content:
          application/json:
            schema:
              $ref: schemas.yaml#/MySchema
      responses:
        default:
          description: no reply
          content: {}

schemas.yaml

MySchema:
  description: A simple data structure
  propertiers:
    id:
      description: a unique number
      type: integer
      format: int32
    name:
      description: a name
      type: string

Generation Details

N/A

Steps to reproduce

What I expect from validate:

$ java -jar ./openapi-generator-cli.jar validate -i api_standalone.yaml
Validating spec (api_standalone.yaml)
Errors:
        - attribute paths.'/hello'(post).requestBody.content.'application/json'.schema.propertiers
          is unexpected

[error] Spec has 1 errors.

What I get when using the reference.

$ java -jar ./openapi-generator-cli.jar validate -i api_with_ref.yaml
Validating spec (api_with_ref.yaml)
[main] WARN  io.swagger.v3.parser.OpenAPIV3Parser - Exception while resolving:
java.lang.NullPointerException: null
        at io.swagger.v3.parser.util.OpenAPIDeserializer.getSchema(OpenAPIDeserializer.java:2542)
        at io.swagger.v3.parser.ResolverCache.loadRef(ResolverCache.java:159)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:89)
        at io.swagger.v3.parser.processors.SchemaProcessor.processReferenceSchema(SchemaProcessor.java:213)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchema(SchemaProcessor.java:37)
        at io.swagger.v3.parser.processors.RequestBodyProcessor.processRequestBody(RequestBodyProcessor.java:47)
        at io.swagger.v3.parser.processors.OperationProcessor.processOperation(OperationProcessor.java:45)
        at io.swagger.v3.parser.processors.PathsProcessor.processPaths(PathsProcessor.java:84)
        at io.swagger.v3.parser.OpenAPIResolver.resolve(OpenAPIResolver.java:49)
        at io.swagger.v3.parser.OpenAPIV3Parser.resolve(OpenAPIV3Parser.java:175)
        at io.swagger.v3.parser.OpenAPIV3Parser.readContents(OpenAPIV3Parser.java:154)
        at io.swagger.v3.parser.OpenAPIV3Parser.readLocation(OpenAPIV3Parser.java:89)
        at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:16)
        at org.openapitools.codegen.cmd.Validate.execute(Validate.java:52)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas.yaml#/MySchema
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas.yaml#/MySchema
Exception in thread "main" java.lang.NullPointerException
        at org.openapitools.codegen.cmd.Validate.lambda$execute$2(Validate.java:76)
        at java.base/java.lang.Iterable.forEach(Iterable.java:75)
        at org.openapitools.codegen.cmd.Validate.execute(Validate.java:75)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

Related issues/PRs

Might be related to #10321

Suggest a fix

Reproduce the failure with api_with_ref.yaml and schemas.yaml using the validate command, then inspect Validate.java and the reported OpenAPIDeserializer.java path. The fix is done when an external schema containing the misspelled attribute produces a readable validation error rather than a NullPointerException, while the standalone case remains covered.