Я пытаюсь преобразовать действие карты Google для работы внутри фрагмента, который будет вызываться в NavigationView. Карта показывает, но когда я использовал кодgetMapAsync

mFragment = (SupportMapFragment) getChildFragmentManager().findFragmentById(R.id.map);
mFragment.getMapAsync(this);

Я получил сообщение об ошибке

E/AndroidRuntime: FATAL EXCEPTION: main
E/AndroidRuntime: java.lang.NullPointerException
E/AndroidRuntime: at com.sample.samplemap.samplemapFragment.onCreate(samplemapFragment.java:32)
E/AndroidRuntime: at android.support.v4.app.Fragment.performCreate(Fragment.java:1942)
E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1040)
E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1259)
E/AndroidRuntime: at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1624)
E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
E/AndroidRuntime: at android.os.Handler.handleCallback(Handler.java:725)
E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:92)
E/AndroidRuntime: at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5306)
E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
E/AndroidRuntime: at dalvik.system.NativeStart.main(Native Method)

samplemapFragment.java

package  com.sample.samplemap;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;

public class samplemapFragment extends Fragment implements OnMapReadyCallback {
    GoogleMap mGoogleMap;
    SupportMapFragment mFragment;

    public samplemapFragment() {
        Required empty public constructor
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mFragment = (SupportMapFragment) getChildFragmentManager().findFragmentById(R.id.map);         
        mFragment.getMapAsync(this);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        //Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_sample_map, container, false);
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {

        mGoogleMap = googleMap;
        mGoogleMap.setMyLocationEnabled(true);
    }
}

fragment_sample_map.xml

<RelativeLayout
android:layout_height="match_parent"
android:layout_width="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android"
tools:context="com.sample.samplemap.samplemapFragment"
xmlns:tools="http://schemas.android.com/tools">
    <fragment xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:map="http://schemas.android.com/apk/res-auto" android:layout_width="384dp"
    android:layout_height="300dp" android:id="@+id/map"
    android:name="com.google.android.gms.maps.SupportMapFragment"
    android:layout_marginTop="44dp"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true" /> 
</RelativeLayout>