Google Maps Android API NullPointerException

good afternoon. The problem is that when I run the application in which I have a MapFragment, if I do not write code, it works perfectly but if I write the code to have a reference to a GoogleMap, I get an error "NullPointerException", I do not know what it could be, some idea ?

Here I leave the summary code, with the important parts, the failure it gives me when it is in the line of getMapAsync (this);

public class EventCardViewActivity extends AppCompatActivity implements OnMapReadyCallback{

private GoogleMap googleMap;

MapFragment mapFragment = (MapFragment) getFragmentManager()
            .findFragmentById(R.id.mapFragment);

    mapFragment.getMapAsync(this);


@Override
public void onMapReady(GoogleMap map){
    googleMap = map;
}

Here is the exact fault:

08-09 14:51:22.754 8627-8627/eventoslpa.com.eventoslpa_final E/AndroidRuntime: FATAL EXCEPTION: main
                                                                           Process: eventoslpa.com.eventoslpa_final, PID: 8627
                                                                           java.lang.RuntimeException: Unable to start activity ComponentInfo{eventoslpa.com.eventoslpa_final/eventoslpa.com.eventoslpa_final.activity.EventCardViewActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.maps.MapFragment.getMapAsync(com.google.android.gms.maps.OnMapReadyCallback)' on a null object reference
                                                                               at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2750)
                                                                               at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811)
                                                                               at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                                               at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528)
                                                                               at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                               at android.os.Looper.loop(Looper.java:154)
                                                                               at android.app.ActivityThread.main(ActivityThread.java:6316)
                                                                               at java.lang.reflect.Method.invoke(Native Method)
                                                                               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
                                                                               at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
                                                                            Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.maps.MapFragment.getMapAsync(com.google.android.gms.maps.OnMapReadyCallback)' on a null object reference
                                                                               at eventoslpa.com.eventoslpa_final.activity.EventCardViewActivity.onCreate(EventCardViewActivity.java:85)
                                                                               at android.app.Activity.performCreate(Activity.java:6757)
                                                                               at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
                                                                               at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703)
                                                                               at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811) 
                                                                               at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                                               at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528) 
                                                                               at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                               at android.os.Looper.loop(Looper.java:154) 
                                                                               at android.app.ActivityThread.main(ActivityThread.java:6316) 
                                                                               at java.lang.reflect.Method.invoke(Native Method) 
                                                                               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872) 
                                                                               at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)