dart generator crashes when encountering enums containing only null

Description

dart generator throws NPE when encountering enums containing only null while other client generators (java) process them just fine.

Swagger-codegen version

3.0.24-SNAPSHOT
(built from 2d0f7a9e1b61a470039ea580a4d3024b1da92188 (HEAD -> 3.0.0, origin/3.0.0))

Swagger declaration file content or url

components:
  schemas:
    NullEnum:
        enum:
        - null

this seems to be what caused the bug, I can't include the entire spec.
output:

Exception in thread "Thread-0" java.lang.RuntimeException: Could not process model 'NullEnum'.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:779)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:378)
        at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: java.lang.NullPointerException
        at io.swagger.codegen.v3.generators.dart.DartClientCodegen.buildEnumFromValues(DartClientCodegen.java:390)
        at io.swagger.codegen.v3.generators.dart.DartClientCodegen.postProcessModelsEnum(DartClientCodegen.java:364)
        at io.swagger.codegen.v3.generators.dart.DartClientCodegen.postProcessModels(DartClientCodegen.java:355)
        at io.swagger.codegen.v3.DefaultGenerator.processModels(DefaultGenerator.java:1053)
        at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:375)
        ... 3 more

Command line used for generation

java -jar ~/Documents/projects/swagger-codegen/modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -l dart -i $MYSPEC -o ~/Documents/projects/dart-apiclientswagger/

Start in modules/swagger-codegen/src/main/java/io/swagger/codegen/v3/generators/dart/DartClientCodegen.java, especially buildEnumFromValues and postProcessModelsEnum. Run the provided generation command with the minimal NullEnum YAML; done means the Dart generator processes an enum containing only null without the reported NullPointerException.