java.lang.NullPointerException with vague log

[nkuehnle]

I'm receiving a nullpointerexception pretty early on in the build process, using a local openapi.json file. This seems unrelated to #32 as there are no nullable enums (but are non-nullable enums).
openapi: 3.1.0
fastapi: 0.1.0

Exception in thread "main" java.lang.NullPointerException
        at java.util.HashSet.<init>(HashSet.java:118)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:408)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:453)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:407)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)