java.lang.NullPointerException when encountering nullable enums

[relsunkaev]

Description

Cannot generate models with nullable enums. The normal openapi-generator tool with the default python generator does not encounter this error (tried with both v4.1.2 and v.5.2.1).

Minimal Reproducible Example

openapi: 3.0.0
info:
  description: TestAPI
  version: "0.0.1"
  title: MRE API Definition
paths:
  /inventory:
    get:
      responses:
        '200':
          description: result
          content:
            application/json:
              schema:
                type: object
                properties:
                  value:
                    type: string
                    nullable: true
                    enum:
                    - value1
                    - value2
                    - null

Traceback

Storing intermediate outputs in /Users/user/repos/personal/fastapi_client/tmp.TvRoVuZrA; it will be removed if generation is successful
[main] INFO  o.o.codegen.DefaultGenerator - Generating with dryRun=false
[main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: python (client)
[main] INFO  o.o.codegen.DefaultGenerator - Generator 'python' is considered experimental.
[main] INFO  o.o.c.l.AbstractPythonCodegen - Environment variable PYTHON_POST_PROCESS_FILE not defined so the Python code may not be properly formatted. To define it, try 'export PYTHON_POST_PROCESS_FILE="/usr/local/bin/yapf -i"' (Linux/Mac)
[main] INFO  o.o.c.l.AbstractPythonCodegen - NOTE: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).
[main] INFO  o.o.c.l.PythonLegacyClientCodegen - Environment variable PYTHON_POST_PROCESS_FILE not defined so the Python code may not be properly formatted. To define it, try 'export PYTHON_POST_PROCESS_FILE="/usr/local/bin/yapf -i"' (Linux/Mac)
[main] INFO  o.o.c.l.PythonLegacyClientCodegen - NOTE: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).
[main] INFO  o.o.c.languages.PythonClientCodegen - generateAliasAsModel is hard coded to true in this generator. Alias models will only be generated if they contain validations or enums
[main] INFO  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
[main] INFO  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
Exception in thread "main" java.lang.RuntimeException: Could not generate model 'inline_response_200'
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:544)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:899)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:441)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.NullPointerException: Null context for variable 'this' on line 25
	at com.samskivert.mustache.Template.getValueIn(Template.java:288)
	at com.samskivert.mustache.Template.getValue(Template.java:224)
	at com.samskivert.mustache.Template.getValueOrDefault(Template.java:279)
	at com.samskivert.mustache.Mustache$VariableSegment.execute(Mustache.java:802)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:866)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:881)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:870)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:866)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$InvertedSegment.execute(Mustache.java:906)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:881)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:866)
	at com.samskivert.mustache.Template.executeSegs(Template.java:157)
	at com.samskivert.mustache.Template.execute(Template.java:134)
	at com.samskivert.mustache.Template.execute(Template.java:125)
	at org.openapitools.codegen.templating.MustacheEngineAdapter.compileTemplate(MustacheEngineAdapter.java:65)
	at org.openapitools.codegen.TemplateManager.write(TemplateManager.java:163)
	at org.openapitools.codegen.DefaultGenerator.processTemplateToFile(DefaultGenerator.java:1041)
	at org.openapitools.codegen.DefaultGenerator.generateModel(DefaultGenerator.java:382)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:535)
	... 4 more