I can't understand this error.

import java.io.Console;

public class Prompter {
  private static Game mGame;

  public Prompter(Game game) {
    mGame = game;
  }
  public void play() {
    while(mGame.getRemainingTries() > 0 && !mGame.isSolved) {
      displayProgress();
      promptForGuess();
    }
    if (mGame.isSolved()) {
      System.out.printf("Congratulations,you won with %d tries remaining\n", mGame.getRemainingTries());


    } else {
      System.out.printf("Bummer, the word was %s.  :(\n"
                        mGame.getAnswer());

    }
  }

  public static boolean promptForGuess() {
    Console console = System.console();
    boolean isHit = false;
    boolean isValidGuess = false;
    while (! isValidGuess) {
      String guessAsString = console.readLine("Enter a letter: ");

      try {
        isHit = mGame.applyGuess(guessAsString);
        isValidGuess = true;
      } catch (IllegalArgumentException iae) {
        console.printf("%s. Please try again.\n", iae.getMessage());
      }
  }
    return isHit;
}

  public void displayProgress() {
    System.out.printf("You have %d tries left to solve:  %s\n",
                      mGame.getRemainingTries(),
                      mGame.getCurrentProgress());
  }

}

public class Hangman {

    public static void main(String[] args) {
        // Enter amazing code here:
      Game game = new Game("treehouse");
      Prompter prompter = new Prompter(game);
      prompter.play();
    }

}

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

  public Game(String answer) {
    mAnswer = answer;
    mHits = "";
    mMisses = "";

  }
  public String getAnswer() {
    return mAnswer;
  }

  private char validateGuess(char letter) {
    if(! Character.isLetter(letter)) {
      throw new IllegalArgumentException("A letter is required");
    }
    letter = Character.toLowerCase(letter);
    if (mMisses.indexOf(letter) >= 0 || mHits.indexOf(letter) >= 0) {
      throw new IllegalArgumentException(letter + "has already been guessed");
    }
    return letter;
  }

  public boolean applyGuess (String letters) {
    if (letters.length() == 0) {
      throw new IllegalArgumentException("No letter found");
    }
    return applyGuess(letters.charAt(0));
  }



  public boolean applyGuess(char letter) {
    letter = validateGuess(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();
}

  public String getAnswer() {
    return mAnswer;
  }

  public boolean isSolved() {
    return getCurrentProgress().indexOf('-') == -1;
  }


}