[BUG][Dart] Failed to generate package for Stripe library

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

Both dart and dart-dio fails to generate the code for the stripe openapi.
https://github.com/stripe/openapi/tree/master/openapi

I have created a repo here with the commands. The validation does not return any problems with the file.
https://github.com/jamesblasco/stripe_openapi

openapi-generator version

5.1.0-SNAPSHOT

OpenAPI declaration file content or url

https://raw.githubusercontent.com/jamesblasco/stripe_openapi/main/spec3.yaml

Generation Details

openapi-generator generate -i spec3.yaml -g dart-dio -o stripe_openapi --enable-post-process-file \
 --reserved-words-mappings=File=STPFile,Source=STPSource  \
 --import-mappings=File=STPFile,Source=STPSource  \
 --type-mappings=File=STPFile,Source=STPSource \
 -c openapi_config.json 

Steps to reproduce

Clone https://github.com/jamesblasco/stripe_openapi
Run sh generate.sh

Related issues/PRs

There are some related issues with the Stripe openapi in another languages
#8397 #8398 #7975 #6816 #2968

Error log

  Exception: Cannot invoke "io.swagger.v3.oas.models.media.Schema.getProperties()" because "schema" is null
        at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:1145)
        at org.openapitools.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:1036)
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:561)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:878)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:440)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.media.Schema.getProperties()" because "schema" is null
        at org.openapitools.codegen.DefaultCodegen.fromParameter(DefaultCodegen.java:4349)
        at org.openapitools.codegen.DefaultCodegen.fromOperation(DefaultCodegen.java:3777)
        at org.openapitools.codegen.languages.DartClientCodegen.fromOperation(DartClientCodegen.java:559)
        at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:1113)
        ... 6 more

Clone the linked jamesblasco/stripe_openapi repository and run generate.sh to reproduce the failure. Start with DefaultCodegen.fromParameter, DartClientCodegen.fromOperation, and DefaultGenerator.processOperation, using spec3.yaml and the reported stack trace to follow the null schema. Done means generation succeeds for the supplied Stripe specification without the NullPointerException.