Null Pointer Exception when implementing Notifications with Firebase Messaging

I am implementing notifications in my app using firebase cloud messaging and following the quickstart guide provided.

I am getting a null pointer exeception on the following line of code in the MyFirebaseMessagingService.java code

I suspect the issue may be the Context being used, but I cant see that in the debugging.

NotificationManager notificationManager =
(NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);  

The following is the logcat entry

  java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at android.content.ContextWrapper.getSystemService(ContextWrapper.java:627)
at com.grgsolutions.checkingin.MyFirebaseMessagingService.sendNotification(MyFirebaseMessagingService.java:180)
at com.grgsolutions.checkingin.CheckInListAdapter$2.onClick(CheckInListAdapter.java:218)

The following is the complete MyFireBaseMessagingService.java code

package com.grgsolutions.checkingin;
/**
* Copyright 2016 Google Inc. All Rights Reserved.
* <p>
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* <p>
* http://www.apache.org/licenses/LICENSE-2.0
* <p>
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;
import androidx.core.app.NotificationCompat;
import android.util.Log;
import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;
import androidx.work.OneTimeWorkRequest;
import androidx.work.WorkManager;
/**
* NOTE: There can only be one service in each app that receives FCM messages. If multiple
* are declared in the Manifest then the first one will be chosen.
*
* In order to make this Java sample functional, you must remove the following from the Kotlin messaging
* service in the AndroidManifest.xml:
*
* <intent-filter>
*   <action android:name="com.google.firebase.MESSAGING_EVENT" />
* </intent-filter>
*/
public class MyFirebaseMessagingService extends FirebaseMessagingService {
private static final String TAG = "MyFirebaseMsgService";
private String messageBody;
private Context context;
private String packageName;
private String channelId = "NewCheckIn";
/**
* Called when message is received.
*
* @param remoteMessage Object representing the message received from Firebase Cloud Messaging.
*/
// [START receive_message]
@Override
public void onMessageReceived(RemoteMessage remoteMessage) {
// [START_EXCLUDE]
// There are two types of messages data messages and notification messages. Data messages
// are handled
// here in onMessageReceived whether the app is in the foreground or background. Data
// messages are the type
// traditionally used with GCM. Notification messages are only received here in
// onMessageReceived when the app
// is in the foreground. When the app is in the background an automatically generated
// notification is displayed.
// When the user taps on the notification they are returned to the app. Messages
// containing both notification
// and data payloads are treated as notification messages. The Firebase console always
// sends notification
// messages. For more see: https://firebase.google.com/docs/cloud-messaging/concept-options
// [END_EXCLUDE]
// TODO(developer): Handle FCM messages here.
Log.d(TAG, "From: " + remoteMessage.getFrom());
// Check if message contains a data payload.
if (remoteMessage.getData().size() > 0) {
Log.d(TAG, "Message data payload: " + remoteMessage.getData());
//   if (/* Check if data needs to be processed by long running job */ true) {
//       // For long-running tasks (10 seconds or more) use WorkManager.
//       scheduleJob();
//   } else {
//       // Handle message within 10 seconds
//      handleNow();
//    }
}
// Check if message contains a notification payload.
if (remoteMessage.getNotification() != null) {
Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
}
// Also if you intend on generating your own notifications as a result of a received FCM
// message, here is where that should be initiated. See sendNotification method below.
}
// [END receive_message]
// [START on_new_token]
/**
* Called if InstanceID token is updated. This may occur if the security of
* the previous token had been compromised. Note that this is called when the InstanceID token
* is initially generated so this is where you would retrieve the token.
*/
@Override
public void onNewToken(String token) {
Log.d(TAG, "Refreshed token: " + token);
// If you want to send messages to this application instance or
// manage this apps subscriptions on the server side, send the
// Instance ID token to your app server.
sendRegistrationToServer(token);
}
// [END on_new_token]
/**
* Schedule async work using WorkManager.
*/
/*
private void scheduleJob() {
// [START dispatch_job]
OneTimeWorkRequest work = new OneTimeWorkRequest.Builder(MyWorker.class)
.build();
WorkManager.getInstance().beginWith(work).enqueue();
// [END dispatch_job]
}
/**
* Handle time allotted to BroadcastReceivers.
*/
/*
private void handleNow() {
Log.d(TAG, "Short lived task is done.");
}
*/
/**
* Persist token to third-party servers.
*
* Modify this method to associate the user's FCM InstanceID token with any server-side account
* maintained by your application.
*
* @param token The new token.
*/
private void sendRegistrationToServer(String token) {
// TODO: Implement this method to send token to your app server.
}
/**
* Create and show a simple notification containing the received FCM message.
*
* @param messageBody FCM message body received.
*/
public void sendNotification(Context context, String messageBody) {
Intent intent = new Intent (context,  CheckIn.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
PendingIntent pendingIntent = PendingIntent.getActivity(context, 0 , intent,
PendingIntent.FLAG_ONE_SHOT);
String channelId = context.getResources().getString(R.string.new_check_in_channel_id);
Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
NotificationCompat.Builder notificationBuilder =
new NotificationCompat.Builder(context, channelId)
.setSmallIcon(R.drawable.ic_launcher)
.setContentTitle(context.getResources().getString(R.string.notifications_header))
.setContentText(messageBody)
.setAutoCancel(true)
.setContentIntent(pendingIntent)
.setSound(defaultSoundUri);
NotificationManager notificationManager =
(NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
// Since android Oreo notification channel is needed.
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
NotificationChannel channel = new NotificationChannel(channelId,
"Channel human readable title",
NotificationManager.IMPORTANCE_DEFAULT);
notificationManager.createNotificationChannel(channel);
}
notificationManager.notify(0 /* ID of notification */, notificationBuilder.build());
}
}

The following is the complete AndroidManifest.xml file

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.grgsolutions.checkingin">
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<!-- Set custom default icon. This is used when no icon is set for incoming notification messages.
-->
<meta-data
android:name="com.google.firebase.messaging.default_notification_icon"
android:resource="@drawable/ic_launcher" />
<!-- Set color used with incoming notification messages. This is used when no color is set for the incoming
-->
<meta-data
android:name="com.google.firebase.messaging.default_notification_color"
android:resource="@color/colorAccent" />
<meta-data
android:name="com.google.firebase.messaging.default_notification_channel_id"
android:value="@string/default_notification_channel_id" />
<activity
android:name=".SettingsActivity"
android:label="@string/title_activity_settings"></activity>
<meta-data
android:name="com.google.android.gms.ads.APPLICATION_ID"
android:value="ca-app-pub-3940256099942544~3347511713" />
<activity
android:name=".Emergency"
android:label="@string/title_activity_emergency"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".Contacts"
android:label="@string/title_activity_contacts"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".MapsAndDirections"
android:label="@string/title_activity_maps_and_directions"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".PeepsLocator"
android:label="@string/title_activity_peeps_locator"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".RequestCheckIn"
android:label="@string/title_activity_request_check_in"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".ViewCheckIns"
android:label="@string/title_activity_view_check_ins"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".RoadTrip"
android:label="@string/title_activity_road_trip"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".Commute"
android:label="@string/title_activity_commute"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".Settings"
android:label="@string/title_activity_settings"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".CheckIn"
android:label="@string/title_activity_check_in"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".CheckInNew"
android:label="CheckInNew"
android:theme="@style/AppTheme.NoActionBar" />
<activity
android:name=".MainActivity"
android:label="@string/app_name"
android:theme="@style/AppTheme.NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<service
android:name=".MyFirebaseMessagingService"
android:exported="false">
<intent-filter>
<action android:name="com.google.firebase.MESSAGING_SERVICE"/>
</intent-filter>
</service>
</application>
</manifest>
</details>
# 答案1
**得分**: 1
NotificationManager notificationManager =（NotificationManager）context.getSystemService（Context.NOTIFICATION_SERVICE）
<details>
<summary>英文:</summary>
NotificationManager notificationManager =(NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE)
</details>