[ALL] Boolean cannot be cast to Schema

Description

I'm running into an issue where swagger-codegen fails to generate files for any generation target when referencing an external schema, with a variant of the following error

Exception in thread "Thread-0" java.lang.RuntimeException: Could not process model 'VRI.RegistrationHelper'.Please make sure that your schema is correct!
        at io.swagger.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:363)
        at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:724)
        at io.swagger.codegen.cmd.Generate.run(Generate.java:331)
        at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCastException: java.base/java.lang.Boolean cannot be cast to io.swagger.v3.oas.models.media.Schema

I have no concerns that my external schemas are valid, swagger-editor is able to parse and generate documentation for them, they pass JSON schema validation tools, etc.

This schema is quite complex, and I wish I had a simpler example to give you, but I do not. However, I've linked the schemas and OpenAPI file to this issue.

Swagger-codegen version

3.0.0-SNAPSHOT. Regression status unknown. Has always been a 3.0.0 target.

Swagger declaration file content or url

gist
linked schema 1
linked schema 2

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i vri.yaml -l java -o ./

Steps to reproduce

Run command found in Command line used for generation

Related issues/PRs

None.

Suggest a fix/enhancement

Start with the linked OpenAPI and schema gists, then trace the reported failure through DefaultGenerator.java and Generate.run. Reproduce the command shown with the supplied external schemas and identify where the Boolean is treated as a Schema; done means generation completes without the ClassCastException.