Can't solve this error....

public class Hangman {

    public static void main(String[] args) {
      Game game = new Game("treehouse");
      Prompter prompter = new Prompter(game);
      prompter.play();

    }
}

import java.io.Console;

public class Prompter {
  private Game mGame;

  public Prompter(Game game) {
    mGame = game;
  }
  public void play() {
    while (mGame.getRemainingTries() > 0) {
      displayProgress();
      promptForGuess();
    }
  }

  public boolean promptForGuess() {
    Console console = System.console();
    String guessAsString = console.readLine("Enter a letter: ");
    char guess = guessAsString.charAt(0);
    return mGame.applyGuess(guess);
  }
  public void displayProgress() {
    System.out.printf("You have %d tries to solve:  %s\n", 
                      mGame.getRemainingTries(),
                      mGame.getCurrentProgress());
  }
}

public class Game {
  public static final int MAX_MISSES = 7;
  private String mAnswer;
  private String mHits;
  private String mMisses;
  private String letter;

  public Game(String answer) {
    mAnswer = answer;
  }

  public boolean applyGuess(char letter) {
    boolean isHit = mAnswer.indexOf(letter) >= 0;
    if (isHit) {
      mHits += letter;
    } else {
      mMisses += letter;
    }
    return isHit;
  }

  public String getCurrentProgress() {
    String progress = "";
    for (char letter : mAnswer.toCharArray()) {
      char display = '-';
      if (mHits.indexOf(letter) >= 0) {
        display = letter;
      }
      progress += display;
    }
    return progress;
  }

  public int getRemainingTries() {
    return MAX_MISSES - mMisses.length();
  }
}