Android Ribbit App- signed up first user to parse and then blank screen on emulator happened?

protected EditText mUsername;
protected EditText mPassword;
protected EditText mEmail;
protected Button mSignUpButton;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
    setContentView(R.layout.activity_sign_up);

    mUsername = (EditText)findViewById(R.id.usernameField);
    mPassword = (EditText)findViewById(R.id.passwordField);
    mEmail= (EditText)findViewById(R.id.emailField);
    mSignUpButton = (Button)findViewById(R.id.signupButton);
    mSignUpButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String username = mUsername.getText().toString();
            String password = mPassword.getText().toString();
            String email = mEmail.getText().toString();

            username = username.trim();
            password = password.trim();
            email = email.trim();

            if (username.isEmpty() || password.isEmpty() || email.isEmpty()) {
                AlertDialog.Builder builder = new AlertDialog.Builder(SignUpActivity.this);
                builder.setMessage(R.string.sign_up_error_message)
                        .setTitle(R.string.sign_up_error_title)
                        .setPositiveButton(android.R.string.ok, null);
                AlertDialog dialog = builder.create();
                dialog.show();
            } else {
                //create a new user!
                setProgressBarIndeterminate(true);
                ParseUser newUser = new ParseUser();
                newUser.setUsername(username);
                newUser.setPassword(password);
                newUser.setEmail(email);
                newUser.signUpInBackground(new SignUpCallback() {
                    @Override
                    public void done(ParseException e) {
                        setProgressBarIndeterminate(false);
                        if (e == null) {
                            //success
                            Intent intent = new Intent(SignUpActivity.this, MainActivity.class);
                            intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                            startActivity(intent);
                        }
                        else {
                            AlertDialog.Builder builder = new AlertDialog.Builder(SignUpActivity.this);
                            builder.setMessage(e.getMessage())
                                    .setTitle(R.string.sign_up_error_title)
                                    .setPositiveButton(android.R.string.ok, null);
                            AlertDialog dialog = builder.create();
                            dialog.show();
                        }
                    }
                });
            }
        }
    });
}


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

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * The {@link android.support.v4.view.PagerAdapter} that will provide
 * fragments for each of the sections. We use a
 * {@link FragmentPagerAdapter} derivative, which will keep every
 * loaded fragment in memory. If this becomes too memory intensive, it
 * may be best to switch to a
 * {@link android.support.v13.app.FragmentStatePagerAdapter}.
 */
SectionsPagerAdapter mSectionsPagerAdapter;

/**
 * The {@link ViewPager} that will host the section contents.
 */
ViewPager mViewPager;

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



    ParseAnalytics.trackAppOpened(getIntent());

    //start Login activity
    ParseUser currentUser = ParseUser.getCurrentUser();
    if (currentUser == null) {
        navigateToLogin();
    }
        else {
        Log.i(TAG, currentUser.getUsername());
    }



    // Create the adapter that will return a fragment for each of the three
    // primary sections of the activity.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    ParseObject object;

}

private void navigateToLogin() {
    Intent intent = new Intent(this, LoginActivity.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
    startActivity(intent);
}


@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;
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_logout) {
        ParseUser.logOut();
        navigateToLogin();

    }
    return super.onOptionsItemSelected(item);
}



/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a PlaceholderFragment (defined as a static inner class below).
        return PlaceholderFragment.newInstance(position + 1);
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    //@Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
            case 0:
                return getString(R.string.title_section1).toUpperCase(l);
            case 1:
                return getString(R.string.title_section2).toUpperCase(l);
            case 2:
                return getString(R.string.title_section3).toUpperCase(l);
        }
        return null;
    }
}


/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }
}