NullPointerException

class Game{
    private String answer;
    private String hits;
    private String misses;

 public Game(String answer){
        this.answer = answer;
        }

 public boolean applyGuess(char letter){
        boolean isHit = answer.indexOf(letter) != -1;
        if (isHit){
        hits += letter;
        }
      else{
        misses += letter;
        }
        return isHit;
    }

    public String getCurrentProgress(){
        String progress = "";
        for(char letter : answer.toCharArray()){
        char display = '-';
        if(hits.indexOf(letter) != -1){
        display = letter;
            }
         progress += letter;
        }
        return progress;
    }
}

public class Hangman {

  public static void main(String[] args) {
    // Your incredible code goes here...
    Game game = new Game("treehouse");
        Prompter prompter = new Prompter(game);

        prompter.displayProgress();

            boolean isHit= prompter.promptForGuess();
            if (isHit){
            System.out.print("It's good!!");
            }else{
            System.out.print("Oops,missed...");
            }

  }
}

import  java.util.Scanner;

class Prompter{
    private Game game;

    public Prompter(Game game){
    this.game = game;

    }

    public boolean promptForGuess(){
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a letter: ");
        String guessInput = scanner.nextLine();
        char guess =guessInput.charAt(0);
        return game.applyGuess(guess);
    }

    public void displayProgress(){
        System.out.printf("Try to solve: %s%n",
                                            game.getCurrentProgress());
    }

}