Could not get unknown property 'classesDir' for main classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput.
 Open File

task dist(type: Jar) {
    from files(sourceSets.main.output.classesDir)
    from files(sourceSets.main.output.resourcesDir)
    from {configurations.compile.collect {zipTree(it)}}
    from files(project.assetsDir);

    manifest {
        attributes 'Main-Class': project.mainClassName
    }
}