Nullpointer exception in swagger-codegen with java

Description

When trying to do java codegen with swagger cli, I'm getting a NPE
Swagger json is valid one, has validated using swagger UI

Error log

Exception in thread "Thread-1" java.lang.NullPointerException
at io.swagger.v3.parser.converter.SwaggerConverter.convert(SwaggerConverter.java:849)
at io.swagger.v3.parser.converter.SwaggerConverter.convert(SwaggerConverter.java:827)
at io.swagger.v3.parser.converter.SwaggerConverter.lambda$convert$11(SwaggerConverter.java:1049)
at java.util.LinkedHashMap.forEach(LinkedHashMap.java:684)
at io.swagger.v3.parser.converter.SwaggerConverter.convert(SwaggerConverter.java:1048)
at io.swagger.v3.parser.converter.SwaggerConverter.convert(SwaggerConverter.java:212)
at io.swagger.v3.parser.converter.SwaggerConverter.readLocation(SwaggerConverter.java:90)
at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:19)
at io.swagger.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:433)
at io.swagger.codegen.cmd.Generate.run(Generate.java:310)
at java.lang.Thread.run(Thread.java:748)

Swagger-codegen version

Tried in both 3.0.0-rc0 and 3.0.0-rc1

Swagger declaration file content or url

Sorry, the swagger cannot be pasted here due to privacy reasons

Command line used for generation

I have downloaded the swagger file as a json into my local filesystem
$ java -jar swagger-codegen-cli.jar generate -i swagger.json -l java -o /tmp

Steps to reproduce

Downloaded the 3.0.0-rc0 and 3.0.0-rc1 sources from github, and did a local build in Mac, to get the swagger-codegen-cli binary
Then executed following
$ java -jar swagger-codegen-cli.jar generate -i swagger.json -l java -o /tmp

Related issues/PRs

Suggest a fix/enhancement

Start at SwaggerConverter.java:849 and trace the call from CodegenConfigurator.toClientOptInput in the reported CLI command. Reproduce with the same swagger.json if it can be obtained; completion means Java generation no longer throws the reported NullPointerException, but the private declaration currently prevents direct reproduction.