1.kt

class PluginImpl : JavaPlugin() {
  override fun onLoad() { instance = this }
  
  companion object {
    lateinit var instance : PluginImpl
      private set
  }
}

2.kt

class PluginImpl : JavaPlugin() {
  companion object {
    val instance by lazy {
      JavaPlugin.getPlugin(PluginImpl::class.java)!!
    }
  }
}

3.kt

// Only possible in Kotlin 1.2
lateinit var main : PluginImpl
class PluginImpl : JavaPlugin() {
  override fun onLoad() { main = this }
}