Attempt to invoke virtual method 'boolean android.location.LocationManager.isProviderEnabled(java.lang.String)' on a null object reference

Question

i'm getting this error and don't know why i have this.

"Attempt to invoke virtual method 'boolean android.location.LocationManager.isProviderEnabled(java.lang.String)' on a null object reference"

ActivityMain.java

import android.Manifest;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationManager;
import android.provider.Settings;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
   private static final int REQUEST_LOCATION = 1;
   Button btnGetLocation;
   TextView showLocation;
   LocationManager locationManager;
   String latitude, longitude;
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      ActivityCompat.requestPermissions( this,
      new String[] {Manifest.permission.ACCESS_FINE_LOCATION}, REQUEST_LOCATION);
      showLocation = findViewById(R.id.showLocation);
      btnGetLocation = findViewById(R.id.btnGetLocation);
      btnGetLocation.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            location nManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
            if (!locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER)) {
               OnGPS();
            } else {
               getLocation();
            }
         }
      });
   }
   private void OnGPS() {
      final AlertDialog.Builder builder = new AlertDialog.Builder(this);
      builder.setMessage("Enable GPS").setCancelable(false).setPositiveButton("Yes", new  DialogInterface.OnClickListener() {
         @Override
         public void onClick(DialogInterface dialog, int which) {
            startActivity(new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS));
         }
      }).setNegativeButton("No", new DialogInterface.OnClickListener() {
         @Override
         public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
         }
      });
      final AlertDialog alertDialog = builder.create();
      alertDialog.show();
   }
   private void getLocation() {
      if (ActivityCompat.checkSelfPermission(
         MainActivity.this,Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(
            MainActivity.this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
               ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, REQUEST_LOCATION);
       } else {
         Location locationGPS = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
         if (locationGPS != null) {
            double lat = locationGPS.getLatitude();
            double longi = locationGPS.getLongitude();
            latitude = String.valueOf(lat);
            longitude = String.valueOf(longi);
            showLocation.setText("Your Location: " + "\n" + "Latitude: " + latitude + "\n" + "Longitude: " + longitude);
         } else {
            Toast.makeText(this, "Unable to find location.", Toast.LENGTH_SHORT).show();
        }
      }
   }
}

If someone know how to resolve this problem it would help me a lot.

Thank you for your future answer. =)