[BUG] NullPointerException in parser when referencing invalid schema

[mildsunrise]

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

Minor bug report, you can close it if you like.
It would be nice if the validation error was reported correctly (like usual) instead of NPE.

openapi-generator version

tested with 5.0.0, 5.2.1, 6.0.0-SNAPSHOT

OpenAPI declaration file content or url

test.yaml:

openapi: 3.0.0
info:
  description: test
  version: test
  title: test
paths:
  /test:
    $ref: 'other.yaml#/test'

other.yaml:

test:
  get:
    responses:
      '200':
        description: test
        content:
          application/json:
            schema:
              type: object
              properties:
                prop1:
                  type: array
                prop2:
                  type: date

Steps to reproduce

then, java -jar openapi-generator-cli.jar generate -i test.yaml -g java gives an NPE when trying to access items:

[main] WARN  io.swagger.v3.parser.OpenAPIV3Parser - Exception while resolving:
java.lang.NullPointerException: null
        at io.swagger.v3.parser.processors.SchemaProcessor.processArraySchema(SchemaProcessor.java:196)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchemaType(SchemaProcessor.java:47)
        at io.swagger.v3.parser.processors.SchemaProcessor.processPropertySchema(SchemaProcessor.java:117)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchemaType(SchemaProcessor.java:54)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchema(SchemaProcessor.java:39)
        at io.swagger.v3.parser.processors.ResponseProcessor.processResponse(ResponseProcessor.java:52)
        at io.swagger.v3.parser.processors.OperationProcessor.processOperation(OperationProcessor.java:67)
        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.config.CodegenConfigurator.toContext(CodegenConfigurator.java:523)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:433)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Exception in thread "main" java.lang.NullPointerException
        at org.openapitools.codegen.utils.ModelUtils.lambda$getUnusedSchemas$1(ModelUtils.java:180)
        at org.openapitools.codegen.utils.ModelUtils.visitSchema(ModelUtils.java:325)
        at org.openapitools.codegen.utils.ModelUtils.visitSchema(ModelUtils.java:372)
        at org.openapitools.codegen.utils.ModelUtils.visitContent(ModelUtils.java:306)
        at org.openapitools.codegen.utils.ModelUtils.visitPathItem(ModelUtils.java:254)
        at org.openapitools.codegen.utils.ModelUtils.visitOpenAPI(ModelUtils.java:231)
        at org.openapitools.codegen.utils.ModelUtils.getUnusedSchemas(ModelUtils.java:179)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:537)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:433)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

Putting it in a single schema produces the correct output, indicating that there's a validation error and where.

[mildsunrise]

Removing prop2 gives a slightly better output:

[main] WARN  io.swagger.v3.parser.OpenAPIV3Parser - Exception while resolving:
java.lang.NullPointerException: null
        at io.swagger.v3.parser.processors.SchemaProcessor.processArraySchema(SchemaProcessor.java:196)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchemaType(SchemaProcessor.java:47)
        at io.swagger.v3.parser.processors.SchemaProcessor.processPropertySchema(SchemaProcessor.java:117)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchemaType(SchemaProcessor.java:54)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchema(SchemaProcessor.java:39)
        at io.swagger.v3.parser.processors.ResponseProcessor.processResponse(ResponseProcessor.java:52)
        at io.swagger.v3.parser.processors.OperationProcessor.processOperation(OperationProcessor.java:67)
        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.config.CodegenConfigurator.toContext(CodegenConfigurator.java:523)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:433)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 0
Errors: 
        -null

        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:546)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:433)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

[oldicestick]

Hi,

I am able to reproduce the issue, I can apply some changes so it throws a more meaningful exception instead of a general NPE.