问如何解决java.lang.NullPointerException:在android中

EN

package com.example.map;

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.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

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) {
               LocationManager 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(String.format("%s\n%s%s\n%s%s", getString(R.string.your_location), getString(R.string.latitude), latitude, getString(R.string.longitude), longitude));
            } else {
                Toast.makeText(this, "Unable to find location.", Toast.LENGTH_SHORT).show();
            }
        }
    }
}


>     activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<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:orientation="vertical"
    android:gravity="center"
    tools:context=".MainActivity">
    <TextView
        android:id="@+id/showLocation"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Location"
        android:textSize="24sp" />
    <Button
        android:id="@+id/btnGetLocation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Get Location" />
</LinearLayout>