[Java] Failed to generate Java SDK

Description

Failed to generate java sdk from Mattersmost Open API:

• blob:https://api.mattermost.com/855a66b2-c0c7-4a06-85d9-302f4639f5a1

16:09:32.655 [Thread-0] INFO  i.s.c.v.i.CodegenIgnoreProcessor - No .swagger-codegen-ignore file found.
Exception in thread "Thread-0" java.lang.RuntimeException: Could not process model 'PluginStatus'.Please make sure that your schema is correct!
	at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:386)
	at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:788)
	at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:388)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.NullPointerException
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.lambda$processPropertySchemaTypes$0(DefaultCodegenConfig.java:1726)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.processPropertySchemaTypes(DefaultCodegenConfig.java:1726)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromProperty(DefaultCodegenConfig.java:1587)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.addVars(DefaultCodegenConfig.java:3159)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.addVars(DefaultCodegenConfig.java:3129)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.addVars(DefaultCodegenConfig.java:3117)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromModel(DefaultCodegenConfig.java:1479)
	at io.swagger.codegen.v3.generators.java.AbstractJavaCodegen.fromModel(AbstractJavaCodegen.java:954)
	at io.swagger.codegen.v3.DefaultGenerator.processModels(DefaultGenerator.java:1019)
	at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:375)
	... 3 more

Swagger-codegen version

3.0.41

Swagger declaration file content or url

Command line used for generation

java -jar swagger-codegen-cli-3.0.41.jar generate -l java \
  -i ./mattersmost-openapi.json \
  -o ./mattersmost-client \
  --group-id=com.github.luozhouyang \
  --artifact-id=mattersmost-client \
  --artifact-version=0.1.0-SNAPSHT \
  --model-package=com.github.luozhouyang.mattersmost.client.model \
  --api-package=com.github.luozhouyang.mattersmost.client.api \
  --invoker-package=com.github.luozhouyang.mattersmost.client.invoker \
  --library=retrofit2

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Start by obtaining the Mattermost OpenAPI document from the linked blob and reproducing the command with swagger-codegen-cli 3.0.41. Inspect DefaultCodegenConfig.processPropertySchemaTypes at line 1726 and the PluginStatus model path shown in the stack trace; done means Java SDK generation completes without the NullPointerException.