i get a NullPointerException At Class Forum line 14 but i can't seem to find the mistake i made

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

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

  public String getTitle() {
    return mTitle;
  }
  public String getDescription(){
    return mDescription;
  }
  public void setDescription(String name){
    mDescription = name;
  }

  // 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 void setFirstName(String name){
    mFirstName = name;
  }
  public void setLastName(String name){
    mLastName= name;
  }
}

public class Forum {
  private String mTopic;

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

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

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("Brecht","Philips");
    // Add the author, title and description
    ForumPost post = new ForumPost(author,"hello","it");
    forum.addPost(post);
  }
}