I'm getting an error that is not allowing me to print out a sorting list

Question

I am trying to print out a list of words from a given text, that will print it out in alphabetical order and then print out the frequencies of each word along side it.

I cannot it get to print out because I am getting this error:

Exception in thread "main" java.lang.NullPointerException
    at java.lang.String.compareTo(String.java:1155)
    at List.insert(Count2.java:101)
    at Count2.main(Count2.java:17)

This is my code:

  import java.util.Scanner;

    public class Count2
    {
        static Scanner stdin = new Scanner(System.in);

    public static void main(String[] args)
    {
    String word;
    List sortedList = new List();
    word = nextWord();

    while(word != null)
    {
        sortedList.insert(word);
        word = nextWord();
    }
    sortedList.print();
    }



    private static String nextWord()
    {

    if(stdin.hasNext())
    {
        String word = stdin.next();
        word = word.toLowerCase();
        int start = 0;
        int end = word.length();
        for(int c = 0; c < word.length(); ++c)
        {
            if(Character.isLetter(word.charAt(c)) || word.charAt(c) == '-') 
            {
                start = c;
                break;
            }
        }
        for(int n = start; n < word.length(); ++n)
        {
            if(!(Character.isLetter(word.charAt(n)) || word.charAt(n) == '-'))
            {
                end = n;
                break;
            }
        }
        return word.substring(start,end);
    }

    else return null;


        } // nextWord


    } // end Count2

class List {
    static class Node {
        String word;
        int count;
        Node next;

        Node(String word, Node next) {
            word = word;
            next = next;
            count = 1; //the first time you add a word
        }
    }
    private Node first;
    private int numWords;

    public List() { //make an empty list
        first = null;
        numWords = 0;
    }

    public void insert(String word) 
    {
        if(first == null)
        {
            Node newNode;
            newNode= addNode(word, null);
            first = newNode;
        }
        else if (word.equals(first.word))
        {
            first.count++;
        }

        else
        {
            Node newNode;
            Node current;
            current = first;
            Node previous;
            previous = null;
            int i = word.compareTo(current.word);

            while((i > 0) && (current.next != null))
            {
                previous = current;
                current = current.next;
                i = word.compareTo(current.word);
            }

            if (( i > 0 && current.next == null))
            {
                newNode = addNode(word, null);
                current.next = newNode;
            }

            else if ( i == 0)
            { 
                current.count++;
            }

            else
            {
                newNode = addNode(word, current);

                if( previous == null)
                {
                    first = newNode;
                }

                else
                {
                    previous.next = newNode;
                }
            }

        }

    }

    private Node addNode(String word, Node next)
    {
        Node newNode = new Node(word, next);
        numWords++;
        return newNode;
    }

    public String[] getWords()
    {
        String[] WORDS = new String[numWords];
        Node current = first;
        int m = 0;

        while( current != null)
        {
            WORDS[m] = current.word;
            current = current.next;
            m++;
        }
        return WORDS;
    }

    public int[] getNumbers()
    {
        int [] numbers = new int[numWords];
        Node current = first;
        int m = 0;

        while(current != null)
        {
            numbers[m] = current.count;
            current = current.next;
            m++;
        }
        return numbers;
    }
    public void print() 
    {
        int[] numbers = getNumbers();
        String [] WORDS = getWords();

        System.out.println("Word  \t  \t  Occurs");
        System.out.println("====  \t  \t  ======");

        for( int i = 0; i < numWords; i++)
        {
            System.out.println(WORDS[i] + " \t " + numbers[i]);
        }
    }
}