[C#] Base class with enum field causes NullPointerException

Description

Base class with enum field causes NullPointerException:

Exception in thread "main" java.lang.RuntimeException: Could not process model 'TestNs.TestClass2'.Please make sure that
your schema is correct!
at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:297)
at io.swagger.codegen.cmd.Generate.run(Generate.java:223)
at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:36)
Caused by: java.lang.NullPointerException
at io.swagger.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1176)
at io.swagger.codegen.languages.CSharpClientCodegen.fromModel(CSharpClientCodegen.java:313)
at io.swagger.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1158)
at io.swagger.codegen.languages.CSharpClientCodegen.fromModel(CSharpClientCodegen.java:316)
at io.swagger.codegen.DefaultGenerator.processModels(DefaultGenerator.java:875)
at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:290)
... 2 more

Swagger-codegen version

2.2.1

Swagger declaration file content or url

{
  "swagger": "2.0",
  "info": {
    "version": "v1",
    "title": "TestNs"
  },
  "host": "localhost:80",
  "schemes": [ "https" ],
  "paths": {
    "/Test": {
      "get": {
        "tags": [ "Test" ],
        "summary": "Tests the method.",
        "operationId": "TestMethod",
        "consumes": [ ],
        "produces": [ "application/json", "text/json", "application/xml", "text/xml" ],
        "responses": {
          "200": {
            "description": "OK",
            "schema": { "$ref": "#/definitions/TestNs.TestClass" }
          }
        },
        "deprecated": false
      }
    }
  },
  "definitions": {
    "TestNs.TestClass": {
      "required": [ "$Type" ],
      "type": "object",
      "properties": {
        "Enum": {
          "enum": [ "Value1", "Value2" ],
          "type": "string"
        },
        "$Type": {
          "description": "The discriminator.",
          "type": "string"
        }
      },
      "discriminator": "$Type"
    },
    "TestNs.TestClass2": {
      "type": "object",
      "allOf": [
        { "$ref": "#/definitions/TestNs.TestClass" },
        {
          "type": "object",
          "properties": { }
        }
      ],
      "properties": { }
    }

Command line used for generation

java -jar Tools\swagger-codegen-cli-2.2.1.jar generate -i $swaggerEndpoint -l csharp -D packageName=$projectName -o $clientFolder

Reproduce the failure with the supplied Swagger definition and the stated swagger-codegen CLI command. Start at CSharpClientCodegen.fromModel and DefaultCodegen.fromModel, using the stack trace to follow processing of the inherited enum field. Done means the same base-class and allOf model generates a C# client without a NullPointerException.