Error in nested use of Android Fragment

Each time the new SDK creates an activity, it will automatically generate

public static class PlaceholderFragment extends Fragment

Fragment module. The nested fragment code is as follows:
public static class PlaceholderFragment extends Fragment {static FragmentManager fm;public PlaceholderFragment() {   fm=getChildFragmentManager();}@Override  public void onCreate(Bundle savedInstanceState) {      super.onCreate(savedInstanceState);       }  @Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {View rootView=LayoutInflater.from(getActivity()).inflate(R.layout.tab, null);if(fm.findFragmentByTag("haha")==null){Fragment1 f1=new Fragment1();fm.beginTransaction().add(R.id.frame_tab,f1,"haha").commit();}return rootView;}
The error message is as follows:

In short, the Acitivity is destroyed.
Cause:
Fm = getChildFragmentManager (); it is called in the PlaceholderFragment constructor. At this time, it has not created onCreate. Naturally, it cannot get the Fragment and activity, so it is written
public void onCreate(Bundle savedInstanceState) {      super.onCreate(savedInstanceState);   fm=getChildFragmentManager();    }  

To solve the error, you must pay attention to Fragment and activity lifecycle.