I can't get away from this NullPointerExpection

public class Main {

    public static void main(String[] args) throws Exception {

        System.out.printf("Welcome to Hangman!");
        Game game = new Game("program");
        Prompter prompter = new Prompter(game);
        prompter.play();

    }
}

public class Prompter
{
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
    private  Game game;

    public  Prompter(Game Game)
    {
        game = Game;
    }



    public void play() throws Exception {

        char letter;
        while (game.getRemainingLives() > 0 && !game.isSolved())
        {
            displayProgress();
            promptForGuess();



        }
        if(game.isSolved())
        {
            System.out.printf("Congratulations! You've won the game with %d tries remaining.", game.getRemainingLives());
        }

        else
        {
            System.out.printf("You lose! Game over! The answer is %s", game.Answer());
        }

        System.out.printf("Sorry! You lose. The answer is %s. Game over!", game.getWord());
        System.exit(0);
    }

    public boolean promptForGuess() throws Exception
    {

        boolean isHit = false;
        boolean isValid = false;
        char singuess;

        while(!isValid)
        {
            System.out.printf("Enter a letter: ");
            String guessAsString = bufferedReader.readLine();
            try {
                singuess = guessAsString.charAt(0);
                isHit = game.Guess(singuess);
                isValid = true;
            }

            catch (IllegalArgumentException e)
            {
                System.out.printf("ERROR: %s!", e);
            }
        }


        return  isHit;
    }

    public void displayProgress()
    {
        System.out.printf(" You have %s lives left. Solve this problem: %s", game.getRemainingLives(), game.getCurrentProgress());
    }


}

public class Game {
    public static final int MAX_LIVES = 7;
    private String word;



    private  String hits;
    private  String misses;

    public String getWord() {
        return word;
    }

    private char validateGuess(char letter) throws Exception {
        if(!Character.isLetter(letter))
        {
            throw  new  Exception("That's not a letter! pick a letter");
        }
        letter = Character.toLowerCase(letter);
        if (misses.indexOf(letter) >= 0 || hits.indexOf(letter) >= 0)
        {
            throw new Exception("You already guessed the letter " + letter + ". Pick a letter you haven't chosen");
        }

        return letter;
    }

    public Game(String Word)
    {
        word = Word;
    }

    public boolean Guess(String letters) throws Exception
    {
        if(letters.length() == 0)
        {
            throw new IllegalArgumentException("Your answer is null. Please input");
        }
        char firstLetter = letters.charAt(0);
        return  Guess(firstLetter);


    }

    public String Answer()
    {
        return getWord();
    }

    public boolean Guess(char letter) throws Exception {
        validateGuess(letter);
        boolean isHit = word.indexOf(letter) >= 0;
        if(isHit)
            hits+= letter;
        else
            misses+= letter;

        return  isHit;
    }

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

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

    public int getRemainingLives()
    {


        return  MAX_LIVES - misses.length();
    }
}