firebase subscribeToTopic OnCompleteListener 没有被调用

我正在开发一个 Android 项目，其中用户将订阅 Firebase 主题。我写了如下代码。

FirebaseMessaging.getInstance().subscribeToTopic("update")
            .addOnCompleteListener(new OnCompleteListener<Void>() {
                @Override
                public void onComplete(@NonNull Task<Void> task) {
                    if (!task.isSuccessful()) {
                        Log.d("SUBSCRIPTION","FAILURE");
                    }else{
                        Log.d("SUBSCRIPTION","SUCCESS");
                    }
                }
            });

但问题是代码中的onComplete方法根本没有被调用。它甚至没有显示失败案例。可能是什么问题呢？由于用户没有订阅该主题，因此我无法发送推送通知。这里有更多详细信息...

清单文件：我已经在清单文件的应用程序标签中添加了。

    <service
        android:name=".MyFirebaseMessagingService">
        <intent-filter>
            <action android:name="com.google.firebase.MESSAGING_EVENT"/>
        </intent-filter>
    </service>

摇篮应用程序：

dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
testImplementation 'junit:junit:4.12


implementation 'com.google.firebase:firebase-core:16.0.1'
implementation 'com.google.firebase:firebase-crash:16.0.1'
implementation 'com.google.firebase:firebase-messaging:17.3.0'


}
apply plugin: 'com.google.gms.google-services'

摇篮项目：

  buildscript {
   repositories {
       jcenter()
       mavenCentral()
       google()
   } 
   dependencies {
       classpath 'com.android.tools.build:gradle:3.1.4'

       // NOTE: Do not place your application dependencies here; they belong
       // in the individual module build.gradle files


       classpath 'com.google.gms:google-services:4.0.1'
   }
 }

EDIT:

我已经使用邮递员检查了主题的订阅，如下所示

https://iid.googleapis.com/iid/info/xxxxxxxxxxxxxxxxx_V7dnQnfaT7f8cV-aNdDqrqyIJdI-fbnai0ec1Dyjm0uoJS0i1mxMfemE0iObR0j0f4mN_4Jyn-vSY4BTxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx?details=true
Authorization: key=xxxxxx //server key
method: GET
RESULT:
{
"applicationVersion": "109",
"connectDate": "2018-08-23",
"attestStatus": "NOT_ROOTED",
"application": "in.xxx.xxxxxxx",
"scope": "*",
"authorizedEntity": "45xxxxxxxxx8",
"connectionType": "MOBILE",
"appSigner": "e9a7xxxxxxxxx29fa0105d0xxxxxxxx33cc",
"platform": "ANDROID"
}

上面的结果没有显示任何主题。