java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.os.Bundle.get(java.lang.String)' on a null object reference

Question

I have an activity that has a button that calling a random activity, but I always encounter this error. I'm still learning, so I really have no idea.

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.os.Bundle.get(java.lang.String)' on a null object reference

button = (Button) findViewById(R.id.play);
button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        ArrayList<Class> activityList = new ArrayList<>();
        activityList.add(eleven14.class);
        activityList.add(fourth10.class);
        activityList.add(MainActivity15.class);
        activityList.add(MainActivity16.class);
        activityList.add(nine15.class);
        activityList.add(sixth8.class);
        activityList.add(ten14.class);
        activityList.add(twelve15.class);

        Random generator = new Random();
        int number = generator.nextInt(8) + 1;

        Class activity = null;

        // Here, we are checking to see what the output of the random was
        switch(number) {
            case 1:
                activity = eleven14.class;
                // We are adding the number of the activity to the list
                activityList.remove(eleven14.class);
                break;
            case 2:
                activity = fourth10.class;
                activityList.remove(fourth10.class);
                break;
            case 3:
                activity = MainActivity16.class;
                activityList.remove(MainActivity16.class);
                break;
            case 4:
                activity = nine15.class;
                activityList.remove(nine15.class);
                break;
            case 5:
                activity = sixth8.class;
                activityList.remove(sixth8.class);
                break;
            case 6:
                activity = ten14.class;
                activityList.remove(ten14.class);
                break;
            case 7:
                activity = twelve15.class;
                activityList.remove(twelve15.class);
                break;
            default:
                activity = MainActivity15.class;
                activityList.remove(MainActivity15.class);
                break;
        }
        // We use intents to start activities
        Intent intent = new Intent(getBaseContext(), activity);
        // `intent.putExtra(...)` is used to pass on extra information to the next activity
        intent.putExtra("LIST_A", activityList);
        startActivity(intent);
    }
});

I put this to other activity

ArrayList<Class> activityList = new ArrayList<>();
Bundle extras = getIntent().getExtras();
activityList = (ArrayList<Class>) extras.get("LIST_A");

if (activityList.size() == 0) {
    showDialog();
} else {
    int r = new Random().nextInt(activityList.size());
    Class<? extends Activity> activity = activityList.remove(r);

    Intent intent = new Intent(getBaseContext(), activity);
    intent.putExtra("LIST_A", activityList);
    startActivity(intent);
}