[R] generation fails with a RuntimeException

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

Reproduce the failure with the supplied openapi.txt and the swagger-codegen 3.0.35 CLI command, then inspect DefaultGenerator.processOperations and generateApis around the reported stack-trace lines. Compare the R generation path with the successful Python command; done means the supplied input generates R code without the RuntimeException.