[R] generation fails with a RuntimeException

[oganm]

Description

Trying to use swagger-codegen on this json to generate R code by doing

java -jar swagger-codegen-cli.jar generate -l r -o swagger_gen -i openapi.txt

I get the error

Exception in thread "Thread-1" java.lang.RuntimeException: Could not generate api file for 'Default'
	at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:588)
	at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:791)
	at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:388)
	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:976)
	at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:503)
	... 3 more

The same json can be used to generate python code without issues

java -jar swagger-codegen-cli.jar generate -l python -o swagger_gen -i openapi.txt

Swagger-codegen version

3.0.35

Swagger declaration file content or url

openapi.txt

Command line used for generation

java -jar swagger-codegen-cli.jar generate -l r -o swagger_gen -i openapi.txt

Steps to reproduce

Run the code below

wget https://github.com/swagger-api/swagger-codegen/files/9429783/openapi.txt
wget https://repo1.maven.org/maven2/io/swagger/codegen/v3/swagger-codegen-cli/3.0.35/swagger-codegen-cli-3.0.35.jar -O swagger-codegen-cli.jar
java -jar swagger-codegen-cli.jar generate -l r -o swagger_gen -i openapi.txt

This will result in the error.

However doing

java -jar swagger-codegen-cli.jar generate -l python -o swagger_gen -i openapi.txt

will not

Related issues/PRs

#6995, #8482 Both appear to have the same error

Suggest a fix/enhancement

I am not familiar with what might be failing under the hood but trying other languages like python and java doesn't seem to be causing the same issue