[KOTLIN] Kotlin constructor for NotNull enum properties is not initialized correctly

Description

[KOTLIN] Constructor initialization for data Kotlin class and NotNull enum properties is not initialized correctly.

As result a build will fail with an error: "Classifier <> does not have a companion object, and thus must be initialized here".

openapi-generator version

openapi-generator-cli-4.1.2.jar

Steps to reproduce

package com.org.model

import java.util.Objects

@JsonIgnoreProperties(ignoreUnknown = true)
data class DocumentExchangeAdd (
@get:NotNull
@JsonProperty("DocumentType") var documentType: DocumentExchangeAdd.DocumentType

) {
constructor() : this(DocumentExchangeAdd.DocumentType)

enum class DocumentType(var value: kotlin.String) {

    @JsonProperty("CreateNewSubmission") createNewSubmission("CreateNewSubmission"),

    @JsonProperty("UpdateDocumentStatus") updateDocumentStatus("UpdateDocumentStatus"),

}

}

Suggest a fix

It has to be initialized by some default value, as on example below:
this(DocumentExchangeAdd.DocumentType.createNewSubmission)

However, I am not sure how to decided what is a default value.

Start with the generated Kotlin data class constructor shown in the reproduction and trace the Kotlin model generation path in openapi-generator 4.1.2. Check how NotNull enum properties receive no-argument defaults; done means generated code compiles without the companion-object error and uses a justified default enum value.