Java Null Pointer Exception?

public class ForumPost {
  private User mAuthor;
  private String mTitle;
  private String mDescription;

   public ForumPost(User author, String title, String description){
    User mAuthor = author;
    mTitle = title;
    mDescription = description;

  }

  public User getAuthor() {
    return mAuthor;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getDescription(){
   return mDescription; 
  }

  // TODO: We need to expose the description
}

public class User {
  private String mFirstName;
  private String mLastName;


  public User(String firstName, String lastName) {
    // TODO:  Set the private fields here
    mFirstName = firstName;
    mLastName = lastName;

  }

  public String getFirstName(){
   return mFirstName; 
  }

  public String getLastName(){
    return mLastName;
  }

}

public class Forum {
  private String mTopic;

  public String getTopic() {
      return mTopic;
  }

  public Forum(String topic){
    mTopic = topic;
  }

  public void addPost(ForumPost post) {
      // When all is ready uncomment this...
      System.out.printf("New post from %s %s about %s \n and %s",
                         post.getAuthor().getFirstName(),
                         post.getAuthor().getLastName(),
                         post.getTitle(),
                         post.getDescription());

      }
}

public class Example {

  public static void main(String[] args) {
    System.out.println("Starting forum example...");
    if (args.length < 2) {
       System.out.println("first and last name are required. eg:  java Example Craig Dennis");
    }
     Forum forum = new Forum("Java");
    // Take the first two elements passed args
     User author = new User(args[0], args[1]);
    // Add the author, title and description
     ForumPost post = new ForumPost(author, "Finishing", "Finished Java Objects Course");
     forum.addPost(post);
  }
}