How to make this work?

I'm unable to make this work.
The instruction on this GitHub page are not very helpful.

I configured the following tasks (`tasks.json`):

```
{
"version": "2.0.0",
"tasks": [
{
"label": "backend: start",
"type": "shell",
"command": "./gradlew bootRun",
"group": {
"kind": "build",
"isDefault": true
},
"presentation": {
"reveal": "always",
"panel": "new"
},
"problemMatcher": []
},
{
"label": "backend: build",
"type": "shell",
"command": "./gradlew clean build",
"group": {
"kind": "build",
"isDefault": true
},
"presentation": {
"reveal": "always",
"panel": "new"
},
"problemMatcher": []
}
]
}
```

Both work great.

I also configured a launch configuration (`launch.json`):

```
{
"version": "0.2.0",
"configurations": [
{
"type": "kotlin",
"request": "launch",
"name": "Kotlin Launch",
"projectRoot": "${workspaceFolder}",
"mainClass": "ApiApplicationKt",
"preLaunchTask": "backend: build"
}
]
}
```

In my build.gradle I configured the following options:

```
buildscript {
ext {
kotlin_version = "1.3.0"
...
}

repositories {
mavenCentral()
google()
jcenter()
gradlePluginPortal()
}

dependencies {
....
classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlin_version}")
classpath("org.jetbrains.kotlin:kotlin-allopen:${kotlin_version}")
}
}

apply plugin: "kotlin"
apply plugin: "kotlin-spring"
apply plugin: "org.springframework.boot"
apply plugin: "io.spring.dependency-management"
apply plugin: 'application'

group = "io.realworld"
version = "0.0.1-SNAPSHOT"
description = """api"""
mainClassName = "ApiApplicationKt"
sourceCompatibility = 1.8
targetCompatibility = 1.8

compileKotlin {
kotlinOptions {
jvmTarget = "1.8"
}
}

compileTestKotlin {
kotlinOptions {
jvmTarget = "1.8"
}
}

tasks.withType(JavaCompile) {
options.encoding = "UTF-8"
}

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
kotlinOptions {
jvmTarget = "1.8"
}
}

repositories {
mavenCentral()
google()
jcenter()
gradlePluginPortal()
}

dependencies {
compile group: "org.jetbrains.kotlin", name: "kotlin-gradle-plugin", version: "${kotlin_version}"
compile group: "org.jetbrains.kotlin", name: "kotlin-stdlib-jdk8", version: "${kotlin_version}"
compile group: "org.jetbrains.kotlin", name: "kotlin-reflect", version: "${kotlin_version}"
....
}
```

When I try to launch the launch configuration I only get a

> java.lang.ClassNotFoundException: ApiApplicationKt

Is my configure okay? What else could I have done wrong to get debugging working with kotling and vs code?

Ps. I wish there would be a simple example for `tasks.json`, `launch.json`, ... in the README of this project.