Stack Machine in kotlin

stack_machine.kt

class Stack(val value:Int, val succ:Stack?){
    fun push(i:Int) = Stack(i, this)
    fun apply(op:(Int, Int) -> Int):Stack = succ?.let{
        Stack(op(value, it.value), succ.succ)
    } ?: throw IllegalArgumentException()
}

fun solution(s:String):Int =s.fold<Stack?>(null){ stack , char -> when(char){
    '0','1','2','3','4','5','6','7','8','9' -> stack?.let {it.push(char - '0')} ?: Stack(char- '0', null)
    '+' -> stack?.let { it.apply{ x1, x2 -> x1 + x2 } } ?: throw IllegalArgumentException()
    '*' -> stack?.let { it.apply{ x1, x2 -> x1 * x2 } } ?: throw IllegalArgumentException()
    else -> throw IllegalArgumentException()
}}?.value ?: 0