[Golang] Could not generate api file for 'Beacon'

Description

Exception in thread "Thread-1" java.lang.RuntimeException: Could not generate api file for 'Beacon'
        at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:579)
        at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:782)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:369)
        at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
        at java.util.TreeMap.put(TreeMap.java:563)
        at java.util.TreeSet.add(TreeSet.java:255)
        at java.util.AbstractCollection.addAll(AbstractCollection.java:344)
        at java.util.TreeSet.addAll(TreeSet.java:312)
        at io.swagger.codegen.v3.DefaultGenerator.processOperations(DefaultGenerator.java:968)
        at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:494)
        ... 3 more

Swagger-codegen version

Tried latest unstable, 3.0.27, 3.0.26, 3.0.20. None of these work.

Swagger declaration file content or url

https://github.com/ethereum/eth2.0-APIs/releases/download/v2.0.0/beacon-node-oapi.json

Command line used for generation

swagger-codegen generate -i https://github.com/ethereum/eth2.0-APIs/releases/download/v2.0.0/beacon-node-oapi.json -l go -o /local/out/go

Reproduce the failure with the provided beacon-node-oapi.json URL and command line, then inspect DefaultGenerator.processOperations and DefaultGenerator.generateApis where the stack trace points. Identify the null value that reaches the TreeSet and verify that Go API generation completes successfully for this declaration.