android fragment嵌套fragment出現的問題：no activity

package com.example.fragmentNavigation2.fragment;

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.ContextThemeWrapper;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import com.example.fragmentNavigation2.R;
import com.viewpagerindicator.TabPageIndicator;

import java.lang.reflect.Field;

public class AddressFragment extends Fragment {

    private static final String[] CONTENT = new String[]{"協會簡介", "礦業新聞", "活動與交流", "會員中心"};
    private FragmentManager fragmentManager;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        fragmentManager = getChildFragmentManager();
        final Context contextThemeWrapper = new ContextThemeWrapper(getActivity(), com.viewpagerindicator.R.style.Theme_PageIndicatorDefaults);
        // clone the inflater using the ContextThemeWrapper
        LayoutInflater localInflater = inflater.cloneInContext(contextThemeWrapper);
        View view = localInflater.inflate(R.layout.simple_tabs, container, false);

        FragmentPagerAdapter adapter = new FragmentPagerAdapter(fragmentManager) {

            @Override
            public Fragment getItem(int position) {
                return TestFragment.newInstance(CONTENT[position % CONTENT.length]);
            }

            @Override
            public CharSequence getPageTitle(int position) {
                return CONTENT[position % CONTENT.length].toUpperCase();
            }

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

        ViewPager pager = (ViewPager) view.findViewById(R.id.pager);
        pager.setAdapter(adapter);

        TabPageIndicator indicator = (TabPageIndicator) view.findViewById(R.id.indicator);
        indicator.setViewPager(pager);
        indicator.setFillViewport(true);
        return view;
    }

    /**
     * 這段可以解決fragment嵌套fragment會崩潰的問題
     */
    @Override
    public void onDetach() {
        super.onDetach();
        try {
            //參數是固定寫法
            Field childFragmentManager = Fragment.class.getDeclaredField("mChildFragmentManager");
            childFragmentManager.setAccessible(true);
            childFragmentManager.set(this, null);
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e); } }
}

解決方案為紅色部分，獲取fragmentManager時要用getChildFragmentManager()這個方法。

還要重寫onDetach方法，照着紅色部分寫。