[BUG] oneOf Could not process model

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?

Description

Running openapi-generator-cli via docker on the latest mockserver spec file throws an error

Exception in thread "main" java.lang.RuntimeException: Could not process model 'Expectations'.Please make sure that your schema is correct!
at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:472)
at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:923)
at org.openapitools.codegen.cmd.Generate.run(Generate.java:416)
at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
Caused by: java.lang.NullPointerException
at org.openapitools.codegen.DefaultCodegen.addProperties(DefaultCodegen.java:2003)
at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1847)
at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1166)
at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:467)
... 3 more

Section of file causing the error is

   "oneOf" : [ {
      "$ref" : "#/components/schemas/Expectation"
    }, {
      "type" : "array",
      "additionalProperties" : false,
      "items" : {
        "$ref" : "#/components/schemas/Expectation"
      }
    } ]

Removing the oneOf and just leaving for example the array section fixes the issue and file generation completes successfully.

openapi-generator version

4.2.2 and 4.2.3-SNAPSHOT run via docker

OpenAPI declaration file content or url

https://api.swaggerhub.com/apis/jamesdbloom/mock-server-openapi/5.8.x

Command line used for generation

docker run --rm -v "mockserver:/local" openapitools/openapi-generator-cli:v4.2.2 generate -i https://api.swaggerhub.com/apis/jamesdbloom/mock-server-openapi/5.8.x -c /local/swagger/config.json -g php -o /local

Steps to reproduce

Run docker command above

Related issues/PRs

Suggest a fix

Sorry I only do PHP!

Reproduce the failure with the documented Docker command and inspect DefaultCodegen.addProperties, DefaultCodegen.fromModel, and DefaultGenerator.generateModels using the provided oneOf schema. Verify the generator handles the Expectation model and completes PHP generation without the NullPointerException.