Android studio - fixing "void android.view.view.setvisibility(int)' on a null object reference"

void android.view.View.setVisibility(int)' on a null object reference

</android.support.v4.widget.DrawerLayout>

<android.support.design.widget.FloatingActionButton
                android:id="@+id/fab"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_alignBottom="@+id/textView"
                android:layout_alignParentEnd="true"
                android:layout_gravity="end|bottom"
                android:layout_marginEnd="22dp"
                app:backgroundTint="#114F84"
                app:fabSize="mini"
                android:src="@drawable/ic_add"/>

</android.support.v4.widget.DrawerLayout>

public class home extends AppCompatActivity  {


    private DrawerLayout drawer;
    TextView fadetxt;

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

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                presentActivity(view);
            }
        });

    }

    public void presentActivity(View view) {
        ActivityOptionsCompat options = ActivityOptionsCompat.
                makeSceneTransitionAnimation(this, view, "transition");
        int revealX = (int) (view.getX() + view.getWidth() / 2);
        int revealY = (int) (view.getY() + view.getHeight() / 2);

        Intent intent = new Intent(this, MainActivity.class);
        intent.putExtra(MainActivity.EXTRA_CIRCULAR_REVEAL_X, revealX);
        intent.putExtra(MainActivity.EXTRA_CIRCULAR_REVEAL_Y, revealY);

        ActivityCompat.startActivity(this, intent, options.toBundle());
    }
}

public class MainActivity extends Activity implements View.OnClickListener  {

    public static final String EXTRA_CIRCULAR_REVEAL_X = "EXTRA_CIRCULAR_REVEAL_X";
    public static final String EXTRA_CIRCULAR_REVEAL_Y = "EXTRA_CIRCULAR_REVEAL_Y";

    View rootLayout;

    private int revealX;
    private int revealY;

    // use a compound button so either checkbox or switch widgets work.
    private CompoundButton autoFocus;
    private CompoundButton useFlash;
    private TextView statusMessage;
    private TextView barcodeValue;

    private static final int RC_BARCODE_CAPTURE = 9001;
    private static final String TAG = "BarcodeMain";

    public static final String DATA = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    public static Random RANDOM = new Random();



        public static String randomString(int len) {
            StringBuilder sb = new StringBuilder(len);

            for (int i = 0; i < len; i++) {
                sb.append(DATA.charAt(RANDOM.nextInt(DATA.length())));
            }

            return sb.toString();
        }

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

        statusMessage = (TextView)findViewById(R.id.status_message);
        barcodeValue = (TextView)findViewById(R.id.barcode_value);

        autoFocus = (CompoundButton) findViewById(R.id.auto_focus);
        useFlash = (CompoundButton) findViewById(R.id.use_flash);
        autoFocus.setVisibility(View.INVISIBLE);
        useFlash.setVisibility(View.INVISIBLE);

        findViewById(R.id.read_barcode).setOnClickListener(this);
        TextView alpha = (TextView) findViewById(R.id.auto_focus);
        alpha.setText(randomString(6));

        final Intent intent = getIntent();

        rootLayout = findViewById(R.id.root_layout);

        if (savedInstanceState == null && Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP &&
                intent.hasExtra(EXTRA_CIRCULAR_REVEAL_X) &&
                intent.hasExtra(EXTRA_CIRCULAR_REVEAL_Y)) {
            rootLayout.setVisibility(View.INVISIBLE);

            revealX = intent.getIntExtra(EXTRA_CIRCULAR_REVEAL_X, 0);
            revealY = intent.getIntExtra(EXTRA_CIRCULAR_REVEAL_Y, 0);


            ViewTreeObserver viewTreeObserver = rootLayout.getViewTreeObserver();
            if (viewTreeObserver.isAlive()) {
                viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
                    @Override
                    public void onGlobalLayout() {
                        revealActivity(revealX, revealY);
                        rootLayout.getViewTreeObserver().removeOnGlobalLayoutListener(this);
                    }
                });
            }
        } else {
            rootLayout.setVisibility(View.VISIBLE);
        }
    }

    protected void revealActivity(int x, int y) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            float finalRadius = (float) (Math.max(rootLayout.getWidth(), rootLayout.getHeight()) * 1.1);

            // create the animator for this view (the start radius is zero)
            Animator circularReveal = ViewAnimationUtils.createCircularReveal(rootLayout, x, y, 0, finalRadius);
            circularReveal.setDuration(400);
            circularReveal.setInterpolator(new AccelerateInterpolator());

            // make the view visible and start the animation
            rootLayout.setVisibility(View.VISIBLE);
            circularReveal.start();
        } else {
            finish();
        }
    }

    protected void unRevealActivity() {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            finish();
        } else {
            float finalRadius = (float) (Math.max(rootLayout.getWidth(), rootLayout.getHeight()) * 1.1);
            Animator circularReveal = ViewAnimationUtils.createCircularReveal(
                    rootLayout, revealX, revealY, finalRadius, 0);

            circularReveal.setDuration(400);
            circularReveal.addListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    rootLayout.setVisibility(View.INVISIBLE);
                    finish();
                }
            });


            circularReveal.start();
        }
    }