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>英文翻译</summary>

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

</details>