Why do i get - Exception in thread "main" java.lang.NullPointerException?

import java.util.Arrays;
import java.util.Date;

import com.teamtreehouse.Treet;
public class Example {

  public static void main(String[] args) {
    Treet treet = new Treet(
      "craigsdennis",
      "Want to be famous? Simply tweet about Java and use " +
      "the hashtag #treet. I'll use your tweet in a new " +
      "@treehouse course about data structures.",
      new Date(1421849732000L)
    );
    Treet secondTreet = new Treet(
      "journeytocode",
      "@treehouse makes learning Java sooooo fun! #treet",
      new Date(1421878767000L)
    );

    System.out.printf("This is a new Treet: %s %n", treet);

    System.out.println("The word are:");
      for (String word: treet.getWords()) {
          System.out.println(word);
      }
    Treet[] treets={treet,secondTreet};

    Arrays.sort(treets);

    for(Treet exampleTreet: treets) {
    System.out.println(exampleTreet);
    }

    }
}

package com.teamtreehouse;

import java.util.Date;


public class Treet implements Comparable {
  private String mAuthor;
  private String mDescription;
  private Date mCreationDate;

public Treet (String author, String description, Date creationDate) {
  mAuthor=author;
  mDescription=description;
  mCreationDate=mCreationDate;
}

  @Override  
  public String toString() {
    return String.format("Treet: \" %s \" by %s on %s", mDescription, mAuthor, mCreationDate);
  }

  @Override
  public int compareTo(Object obj) {
    Treet other = (Treet) obj;
    if (equals(other)) {
      return 0;
    }
    int dateCmp=mCreationDate.compareTo(other.mCreationDate);
    if (dateCmp==0) {
      return mDescription.compareTo(other.mDescription);
            }
    return dateCmp;
  }

 public String getAuthor () {
    return mAuthor;
 } 

 public String getDescription () {
    return mAuthor;
 } 

 public Date getCreationDate () {
    return mCreationDate;
 } 
  public String[] getWords() {
   return mDescription.toLowerCase().split("[^\\w#@']+");
  }
  }