CrystalBall Glitch

package com.example.crystalball;

import android.app.Activity;
import android.graphics.drawable.AnimationDrawable;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;

public class MainActivity extends Activity {

    private CrystalBall mCrystalBall = new CrystalBall();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //declare view variables
        final TextView answerLabel = (TextView) findViewById(R.id.textView1);
        Button getAnswerButton = (Button) findViewById(R.id.button1);

        getAnswerButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                String answer = mCrystalBall.getAnAnswer();

                answerLabel.setText(answer);

                animateCrystalBall();
            }
        });
    }

    private void animateCrystalBall(){
        ImageView crystalBallImage = (ImageView) findViewById(R.id.imageView1);
        crystalBallImage.setImageResource(R.drawable.ball_animation);
        AnimationDrawable ballAnimation = (AnimationDrawable) crystalBallImage.getDrawable();
        if(ballAnimation.isRunning()){
            ballAnimation.stop();
        }
        ballAnimation.start();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}