[BUG] generator cannot handle cross referenced schema $ref

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?

• Have you validated the input using an OpenAPI validator (example)?

• What's the version of OpenAPI Generator used?

• Have you search for related issues/PRs?

• What's the actual output vs expected output?
  there should be no warning/error like this

o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas/Pet.yaml#/Pet

Exception in thread "main" java.lang.RuntimeException: Could not process model 'Pet'.Please make sure that your schema is correct!

see more detail below.

• [Optional] Bounty to sponsor the fix (example)

Description

the repro is in the https://github.com/dyno/openapi-ref-issue

cross referenced schema reports error in code generator, (make generate-client)
but shows no problem in swagger-ui-watcher.

openapi-generator version

docker run openapitools/openapi-generator-cli        --version
openapi-generator-cli 5.0.0-SNAPSHOT
  commit : 0cb4c43
  built  : 2020-08-04T04:27:51Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

https://github.com/dyno/openapi-ref-issue

https://github.com/dyno/openapi-ref-issue/blob/master/openapi.yaml

openapi.yaml
schemas/
├── Cat.yaml
├── Dog.yaml
└── Pet.yaml

Generation Details

make generate-client

Steps to reproduce

as above.

[main] INFO  o.o.c.languages.AbstractJavaCodegen - Invoker Package Name, originally not set, is now derived from api package name: openapi.internal
[main] INFO  o.o.c.languages.AbstractJavaCodegen - Processing operation null
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas/Pet.yaml#/Pet
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas/Pet.yaml#/Pet
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas/Pet.yaml#/Pet
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas/Pet.yaml#/Pet
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./schemas/Pet.yaml#/Pet
Exception in thread "main" java.lang.RuntimeException: Could not process model 'Pet'.Please make sure that your schema is correct!
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:475)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:850)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:432)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.NullPointerException
        at org.openapitools.codegen.DefaultCodegen.getAllOfDescendants(DefaultCodegen.java:2817)
        at org.openapitools.codegen.DefaultCodegen.createDiscriminator(DefaultCodegen.java:2871)
        at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:2232)
        at org.openapitools.codegen.languages.AbstractJavaCodegen.fromModel(AbstractJavaCodegen.java:1018)
        at org.openapitools.codegen.languages.JavaClientCodegen.fromModel(JavaClientCodegen.java:717)
        at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1127)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:470)
        ... 4 more

Related issues/PRs

Suggest a fix

Start with the dyno/openapi-ref-issue reproduction, especially openapi.yaml and the schemas/Pet.yaml, Cat.yaml, and Dog.yaml files, then run make generate-client. Trace the warning in ModelUtils and the failure through DefaultCodegen.getAllOfDescendants and DefaultGenerator.generateModels. Done means the cross-referenced schemas generate successfully without the reported warning or exception.