Scala stack example with mutable types

Stack.scala

case class Node(data: Int, next: Option[Node]= None)

class Stack(var head: Node, var size: Int = 1) {

      def push(i: Int): Unit = {
        head = Node(i, Some(head))
        size += 1
      }

      def pop(): Option[Int] = {            
        if(head == null) {
          None
        } else {  
          val r = Some(head.data)
          head = head.next.getOrElse(null)
          r
        }
      }

      def dead = () //throw new NoSuchElementException("Stack.head")
 }

object Stack {
  def apply() = {
    throw new InvalidArgumentException("Stack.head must be provided")
  }

  def apply(i: Int) = {
        new Stack(Node(i))
  }
}