Doubly Linked List addFirst method printing null at the end

Question

I'm fairly new to doubly linked lists and I'm having trouble with one of my methods that is supposed to add a new node to the front of the list. Whenever I'm trying to add to an empty list, it prints out what I wanted to add but it also prints out a null. Here is my code:

public class Box<E> {
private DoubleListNode<E> head;
private DoubleListNode<E> tail;
private int size;

public Box() {
    head = new DoubleListNode<E>();
    tail = head;
    size = 0;
}

private static class DoubleListNode<E> {
    private E myData;
    private DoubleListNode<E> myNext;
    private DoubleListNode<E> myPrev;

    public DoubleListNode() {
        this(null, null, null);
    }
    
    public DoubleListNode(DoubleListNode<E> prev, E data,
            DoubleListNode<E> next) {
        myData = data;
        myNext = next;
        myPrev = prev;
    }
}

public void addFirst(E item) {
    if (item == null) {
        throw new IllegalArgumentException("The parameter"
                + "'item' does not meet the pre conditions.");
    }
    DoubleListNode<E> newNode = new DoubleListNode<E>(null, item, head);
    newNode.myNext = head;
    newNode.myPrev = null;
    if (head != null) {
        head.myPrev = newNode;
    }
    head = newNode;
    size++;
}

 public String toString() {
    if (size == 0) {
        return "[]";
    }
    StringBuilder sb = new StringBuilder("[");
    sb.append(head.myData);
    DoubleListNode<E> temp = head.myNext;
    while (temp != null) {
        sb.append(", ");
        sb.append(temp.myData);
        temp = temp.myNext;
    }
    return sb.append("]").toString();
}   

public static void main(String[] arg) throws FileNotFoundException {
    Box<Integer> dl = new Box<Integer>();
    dl.addFirst(1);
    System.out.println(dl);

}

}

Here in main, I added 1 to the list and it should print [1], but instead it's printing [1, null]