Attempt to invoke virtual method ... on a null object reference [Android Studio]

Question

I have a code to set my ProgressBar with value on the different class but it does not run. i need to set my progressbar with monsterHP then if the monsterHP below 0 it does reset the monster hp and increment the stage.

Heres the code:

public class MainActivity extends Activity {

    MonsterStatus monsterStatus;
    PlayerStatus playerStatus;

    public Button myClick;
    ProgressBar progressBar;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_layout);

        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        myClick = (Button) findViewById(R.id.btnClick);


        progressBar.setProgress(monsterStatus.getMonsterHP());

        myClick.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                monsterStatus.onClick(playerStatus.getClickDamage());
                progressBar.setProgress(monsterStatus.getMonsterHP());
            }
        });

    }
}



public class MonsterStatus {

    private String monsterName;
    private  int monsterHP = 50;
    public boolean isAlive;
    private int currentStage = 1;
    private int headID,bodyID,armsID,legsID;

    public void setMonsterHP(){
        if(currentStage > 10 && currentStage < 21){
            monsterHP = 100;
        }

    }

    public int getMonsterHP(){
        return monsterHP;
    }

    public void onClick(int _clickPower){
        monsterHP -= _clickPower;
        if(monsterHP <= 0){
            currentStage += 1;
            setMonsterHP();
        }
    }
}

What should i do to get the different class variable?