Double linked list, getting a null Exception

[hollybells]

here is code for my assignment. I am getting:

Exception in thread "main" java.lang.NullPointerException
        at SortedStringList.insert(SortedStringList.java:33)
        at TestSortedStringList.main(TestSortedStringList.java:6)

Line 33 is the "while" loop in my insert.

Its supposed to be a double linked list of nodes w/string in each node, alphabetically arranged.

here is the code:

public class SortedStringList  {

    private static class Node {
        public String value;
        public Node next;
        public Node previous;

        public Node(String value, Node next, Node previous) {
            this.value = value;
            this.next = next;
            this.previous = previous;
        }
    }

    private Node first;
    private Node last;

    public SortedStringList() { //default constructor, make an empty list
        this.first = null;
        this.last = null;
    }


    public boolean insert(String s) {
        Node current = first;
        if (this.first == null && this.last == null) {
            this.first = new Node (s, null, null);
        }
        if (this.member(s)) {return false;}


        else while ((current.next != null) || (s.compareToIgnoreCase(current.value) > 0))  {

            current = current.next;
        }

        if (current == last) {
            Node newN = new Node (s, null, last);
            this.last = newN;
            current.next = newN;
            }


        if (current == first) {
            Node newN = new Node (s, first, null);
            first = newN;
            current.previous = newN;
        }

        else {Node newN = new Node(s, current, current.previous);
            current.previous.next = newN;
            current.previous = newN;}
        return true;
        }

    public boolean remove(String s) {
         Node current = first;
        if (this.first == null)
            throw new RuntimeException ("remove called on an empty StringList");
        if (!this.member(s)) {
            return false;
        }

        else while (s.compareToIgnoreCase(current.value) == 0) {
            current = current.next;
        }


        if (current.next == null) {
            this.last = current;
            current.next = null;}
        if (current == this.first) {
            this.first = current.next;
            current.next.previous = null;
        }

        else {  current.previous.next = current.next;
                current.next.previous = current.previous;}
        return true;
    }

    public boolean member(String s) {
        if (first == null) {
            return false;
        }

        Node current = first;
        while (current != null && (s.compareToIgnoreCase(current.value) != 0)) {
            current = current.next;}
        if (current == null) {return false;}
        else {return true;}
    }

    public String toString() {
        String text = "";
        Node current = first;
        while (current != null) {
            text += current.value + "\n";
            current = current.next;
        }
    return text;
    }
}