android - getFragmentById返回null

我正在尝试获取另一个片段中的mapFragment，但是当我调用findFragmentById时，它只返回null，我试图得到它，所以我可以从fragmentmanager中删除它（在onDestroyView方法中）但它会抛出nullpointerexception ，我不知道如何解决这个问题 . 包含 Map 的片段是viewpager的一部分 .

这是我的带有 Map 片段的类：

import android.app.Activity;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.Context;
import android.database.DataSetObserver;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Spinner;
import android.widget.SpinnerAdapter;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.MapFragment;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;

import java.util.ArrayList;
import java.util.List;

import desomer_michael_2app.desomer_michael_eindopdracht_app.Interfaces.ODKortrijkSportLocaties;
import desomer_michael_2app.desomer_michael_eindopdracht_app.Klassen.Sportlocatie;
import desomer_michael_2app.desomer_michael_eindopdracht_app.Util.Connectivity;
import desomer_michael_2app.desomer_michael_eindopdracht_app.Util.WSKortrijkSportlocaties;


/**
 * A simple {@link Fragment} subclass.
 * Activities that contain this fragment must implement the
 * {@link SportStartFragment.OnSportStartFragmentInteractionListener} interface
 * to handle interaction events.
 */
public class SportStartFragment extends Fragment implements ODKortrijkSportLocaties, OnMapReadyCallback {
    private View view;
    private OnSportStartFragmentInteractionListener mListener;

    public SportStartFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        view = inflater.inflate(R.layout.fragment_sportstart, container, false);

        String huidigegebruiker = getActivity().getIntent().getExtras().getString("huidigeGebruiker");
        TextView welkomstTekst = (TextView) view.findViewById(R.id.tV_welkomtekst);
        welkomstTekst.setText("Welkom, " + huidigegebruiker + "!");

        MapFragment map = (MapFragment) getActivity().getFragmentManager().findFragmentById(R.id.map);
        //getActivity().getSupportFragmentManager().putFragment(null, "mapFragm", map);
        map.getMapAsync(this);

        Spinner spinSporten = (Spinner) view.findViewById(R.id.spinSporten);
        spinSporten.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) {

            }
        });

        Connectivity connectie = new Connectivity();
        if(connectie.isConnected(this.getActivity())){
            WSKortrijkSportlocaties webservice = new WSKortrijkSportlocaties();
            webservice.execute(this);
        } else{
            CharSequence toastTekst = "Zorg ervoor dat u verbonden bent met het internet.";
            int duration = Toast.LENGTH_SHORT;
            Toast toast = Toast.makeText(this.getActivity(),toastTekst, duration);
            toast.show();
        }

        return view;
    }

    @Override
    public void onDestroyView(){


            super.onDestroyView();
            Fragment fragm = getActivity().getSupportFragmentManager().findFragmentById(R.id.map);
            android.support.v4.app.FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
            fragmentTransaction.remove(fragm);
            fragmentTransaction.commit();


    }


    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnSportStartFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
        //map.onDestroy();
    }

    @Override
    public void updateKaart(ArrayList<Sportlocatie> sportlocaties) {

    }

    @Override
    public void getSporten(ArrayList<Sportlocatie> sportlocaties) {
        Log.i("SportActivity", "Aantal sportlocaties: " + Integer.toString(sportlocaties.size()));

        ArrayList<String> sporten = new ArrayList<>();

        for(Sportlocatie sl : sportlocaties){
            char hoofdletter = Character.toUpperCase(sl.getSport().charAt(0));
            String hoofdletterString = hoofdletter + sl.getSport().substring(1);

            if(!sporten.contains(hoofdletterString)){
                char first = Character.toUpperCase(sl.getSport().charAt(0));
                String sportHoofdLetter = first + sl.getSport().substring(1);
                sporten.add(sportHoofdLetter);
            }
        }

        ArrayList<String> juisteSporten = new ArrayList();

        for(String sport : sporten){
            if(!juisteSporten.contains("Petanque") && sport.startsWith("Pet")){
                juisteSporten.add("Petanque");
            } else{
                if(!juisteSporten.contains("Duiveltjesveld") && sport.startsWith("Duiv")){
                    juisteSporten.add("Duiveltjesveld");
                } else{
                    if(!sport.startsWith("Pet") && !sport.startsWith("Duiv")) juisteSporten.add(sport);
                }
            }
        }
        Spinner spinnerSporten = (Spinner) view.findViewById(R.id.spinSporten);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this.getActivity(), R.layout.support_simple_spinner_dropdown_item, juisteSporten);
        spinnerSporten.setAdapter(adapter);
    }

    @Override
    public void onMapReady(final GoogleMap googleMap) {

        LocationManager locationManager = (LocationManager) getActivity().getSystemService(Context.LOCATION_SERVICE);
        LocationListener locationListener = new LocationListener() {
            @Override
            public void onLocationChanged(Location location) {
                LatLng lokaal;
                lokaal = new LatLng(location.getLatitude(), location.getLongitude());
                googleMap.moveCamera(CameraUpdateFactory.newLatLngZoom(lokaal, 16));
                googleMap.addMarker(new MarkerOptions().position(lokaal));
            }

            @Override
            public void onStatusChanged(String provider, int status, Bundle extras) {

            }

            @Override
            public void onProviderEnabled(String provider) {

            }

            @Override
            public void onProviderDisabled(String provider) {

            }
        };

        locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0,0, locationListener);

    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p/>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnSportStartFragmentInteractionListener {
        // TODO: Update argument type and name

    }
}

我的片段的xml包含mapfragment：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context="desomer_michael_2app.desomer_michael_eindopdracht_app.SportActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/tV_welkomtekst"
        android:textSize="25dp"
        android:layout_gravity="center"/>

    <TextView
        android:text="@string/sportdropdown_tekst"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="25dp"/>

    <Spinner
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/spinSporten"></Spinner>

    <fragment
        android:layout_width="match_parent"
        android:layout_height="300sp"
        android:id="@+id/map"
        android:name="com.google.android.gms.maps.MapFragment"
        />

    <Button
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/tekst_startWorkout"/>


</LinearLayout>