Double check init

class MyMagicClass private constructor(builder: Builder) {

	class Builder {

	}

	companion object {

		@Volatile
		private var instance: MyMagicClass? = null
	
		fun init(builder: Builder) {
			if (instance != null) {
				throw IllegalStateException("Already init")
			}

			synchronized(this) {
				if (instance == null) {
					instance = MyMagicClass(builder)
				} else {
					throw IllegalStateException("Already init")
				}
			}
		}

		fun instance() = instance ?: throw IllegalStateException("Not init yet")
	}
}