getsupportfragmentmanager().findfragmentbyid(r.id.map) returns null Google map

I try to use google map functionality on my MainActivity I have this null pointer returned by :
getsupportfragmentmanager().findfragmentbyid(r.id.map); ==> return null

SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.mapApi);

mapfragment will be null and when I call it using getMapAsync(this); the app shut down before it starts

 mapFragment.getMapAsync(this); // null pointer variable call getMapAsync 

content_main.xml

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="216dp"
        android:orientation="horizontal">

        <fragment
            android:id="@+id/mapApi"
            android:name="com.google.android.gms.maps.SupportMapFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent">

        </fragment>
    </LinearLayout>

onCreate inside the MainActivity :

public class MainActivity extends AppCompatActivity
        implements  NavigationView.OnNavigationItemSelectedListener , OnMapReadyCallback {



    private GoogleMap map;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
   //setContentView(R.layout.activity_main); didn't work
setContentView(R.layout.content_main); // worked, now i haven't null pointer 
        System.out.println("************************ mapApi:"+R.id.mapApi);

        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.mapApi);


        System.out.println("************************ mapFragment:"+mapFragment);

        if (mapFragment != null) {

            mapFragment.getMapAsync(this);
        }
}

@Override
public void onMapReady(GoogleMap googleMap) {
    map = googleMap;
    LatLng BordjMenaielGare = new LatLng(36.753162, 3.717716);
    LatLng Blida = new LatLng(36.769038, 3.057451);
    map.addMarker(new MarkerOptions().position(BordjMenaielGare).title("Gare Bordj Menaiel "));
    map.addMarker(new MarkerOptions().position(Blida).title("Gare Agha"));
    map.moveCamera(CameraUpdateFactory.newLatLngZoom(BordjMenaielGare, 10));
}

}

here are the files :

the map fragment is on content_main.xml, I included it in app_bar_main.xml, and app_bar_main included in activity_main.xml and I'm calling the map from MainActivity

I searched a lot in StackOverflow but none of the solutions are valid for my case

can someone help me with this?

you need to call:

// add this (supplying the layout for your activity)
setContentView(R.layout.activity); 

after your call to super.onCreate(savedInstanceState);