如何使用视图分页器在android片段选项卡中移动下一页？

我有两个选项卡：登录和注册片段。我在登录选项卡中单击了注册按钮，当我单击时，它应该移动到注册在LoginActivity中使用此方法的片段

public void setCurrentItem(int item, boolean smoothScroll) {
    mPager.setCurrentItem(item, smoothScroll);
}

我在登录片段中得到了该方法，但显示错误。
我的代码如下：

public class LoginActivity extends SherlockFragmentActivity {

    private ViewPager mPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Get the view from activity_main.xml
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.loginpager);
        PagerSlidingTabStrip tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
        tabs.setIndicatorColor(Color.BLUE);
        tabs.setIndicatorHeight(5);
        mPager = (ViewPager) findViewById(R.id.pager);
        ViewPagerAdapter adapter = new ViewPagerAdapter(fm);
        mPager.setAdapter(adapter);
        tabs.setViewPager(mPager);

    }

    public void setCurrentItem(int item, boolean smoothScroll) {
        mPager.setCurrentItem(item, smoothScroll);
    }

    public class ViewPagerAdapter extends FragmentPagerAdapter {

        public ViewPagerAdapter(android.support.v4.app.FragmentManager fm) {
            super(fm);
        }

        private final String[] TITLES = { "LOG IN", "REGISTER" };

        @Override
        public CharSequence getPageTitle(int position) {
            return TITLES[position];
        }

        @Override
        public SherlockFragment getItem(int arg0) {
            switch (arg0) {
            // Open LoginFragment
            case 0:
                LoginFragment loginfragment = new LoginFragment();
                return loginfragment;

            // Open RegisterFragment
            case 1:
                RegisterFragment registerfragment= new RegisterFragment();
                return registerfragment;

            }
        return null;
        }

        @Override
        public int getCount() {
            return TITLES.length;
        }

    } 
}

LoginFragment：

public class LoginFragment extends SherlockFragment implements OnClickListener {

    private LoginActivity loginActivity;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Get the view from fragmenttab3.xml
        View view = inflater.inflate(R.layout.login, container, false);
        SpannableString spannableString = new SpannableString(text);
        spannableString.setSpan(new ForegroundColorSpan(Color.BLACK), 24, 32, 0);
        ClickableSpan clickableSpan = new SpaceAdjust(text) {
            @Override
            public void onClick(View textView) {
                loginActivity = new LoginActivity();
                loginActivity.setCurrentItem(2, true); // here i am getting error. How to swipe to RegisterFragment
            }
        };
        spannableString.setSpan(clickableSpan, 24, 32, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
        dontHaveAccountRegister.setText(spannableString);
        dontHaveAccountRegister.setMovementMethod(LinkMovementMethod.getInstance());
        return view;
    }
}

错误 ：

12-12 00:41:53.906: E/AndroidRuntime(1580): FATAL EXCEPTION: main
12-12 00:41:53.906: E/AndroidRuntime(1580): Process: com.a.b, PID: 1580
12-12 00:41:53.906: E/AndroidRuntime(1580): java.lang.NullPointerException
12-12 00:41:53.906: E/AndroidRuntime(1580):     at com.a.b.LoginActivity.setCurrentItem(LoginActivity.java:148)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at com.a.b.views.LoginFragment$5.onClick(LoginFragment.java:250)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at android.text.method.LinkMovementMethod.onTouchEvent(LinkMovementMethod.java:217)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at android.widget.TextView.onTouchEvent(TextView.java:7724)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at android.view.View.dispatchTouchEvent(View.java:7690)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
12-12 00:41:53.906: E/AndroidRuntime(1580):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)