Intellij Idea 13 UI Designer and automatic Gradle building

I've used the Intellij UI Designer to create forms for a project. Everything works fine when I'm building with idea as it handles compiling the forms for me, but as we recently switched to using Gradle for building it hasn't been possible to produce an executable jar file yet.

My google-fu has led me to several posts that explains that an ant script is needed to compile (eg link, link2, link3 ,and the one i ended on following: link4)

My project is a multi-module setup.

root build.gradle

subprojects {
    apply plugin: 'java'
    apply plugin: 'idea'

    repositories {
        mavenCentral()
    }
}

supproject build.gradle

apply plugin:'application'
mainClassName = "dk.OfferFileEditor.OfferFileEditorProgram"

configurations {
    antTask
}

dependencies {
    compile 'org.json:json:20140107'
    compile project(":Shared:HasOffers Api")

    //dependencies for java2c
    antTask files('../../lib/javac2-13.1.1.jar', '../../lib/asm4-all-13.1.1-idea.jar', '../../lib/forms_rt-13.1.1.jar')
    antTask group: 'org.jdom', name: 'jdom', version: '1.1'
}

task compileJava(overwrite: true, dependsOn: configurations.compile.getTaskDependencyFromProjectDependency(true, 'jar')) {
    doLast {
        println 'using java2c to compile'
        project.sourceSets.main.output.classesDir.mkdirs()
        ant.taskdef name: 'javac2', classname: 'com.intellij.ant.Javac2', classpath: configurations.antTask.asPath
        ant.javac2 srcdir: project.sourceSets.main.java.srcDirs.join(':'),
                classpath: project.sourceSets.main.compileClasspath.asPath,
                destdir: project.sourceSets.main.output.classesDir,
                source: sourceCompatibility,
                target: targetCompatibility,
                includeAntRuntime: false
    }
}

But even though the compilation is successfull, a Nullpointer exception is thrown the first time I try to access one of the fields the UI Designer created. So something is not being compiled correctly.

I'm probably missing some setting, but after unsuccesfully pouring several hours into forums and google I still haven't found any solution.

IntelliJ IDEA 2019.1

I found this issue still exists. It's at least somehow documented now:

If your build actions are delegated to Gradle, GUI Designer will not generate Java source code.

So by disabling the according setting

Build, Execution, Deployment | Build Tools | Gradle | Runner | Delegate IDE build/run actions to gradle

I was able to build and run the project successfully. Note that I didn't need any other settings or additional libraries from the answers above. I let Generate GUI into be set to Binary class files.