Android Studio + Firebase: Sistema não imprime o token

Fazendo o curso Curso Android com Web Service parte 2: mais técnicas de sincronização, ao aplicar os conceitos explanados na aula, não consigo o mesmo resultado.

Mesmo com o FCM informando que está tudo certo, o sistema não obtem o token e imprime no log.

AgendaInstanceIDService.java:

package br.com.alura.agenda.firebase;

import android.util.Log;

import com.google.firebase.iid.FirebaseInstanceId;
import com.google.firebase.iid.FirebaseInstanceIdService;

public class AgendaInstanceIDService extends FirebaseInstanceIdService{

    @Override
    public void onTokenRefresh() {
        // Get updated InstanceID token.
        String refreshedToken = FirebaseInstanceId.getInstance().getToken();
        Log.d("token firebase", "Refreshed token: " + refreshedToken);

        /*
        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(refreshedToken);
    }

    private void sendRegistrationToServer(String refreshedToken) {

    }
}

build.gradle (Module:app):

apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    buildToolsVersion '27.0.3'
    defaultConfig {
        applicationId "br.com.alura.agenda"
        minSdkVersion 15
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    productFlavors {
    }

    packagingOptions {
        exclude 'META-INF/LICENSE'
    }

}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    //noinspection GradleCompatible
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support:cardview-v7:27.1.1'
    implementation 'com.android.support:support-v4:27.1.1'
    implementation 'com.squareup.retrofit2:retrofit:2.1.0'
    implementation 'com.squareup.retrofit2:converter-jackson:2.1.0'
    implementation 'com.squareup.okhttp3:logging-interceptor:3.4.2'
    implementation 'com.google.android.gms:play-services:12.0.1'
    implementation 'com.google.firebase:firebase-messaging:12.0.1'
    implementation 'com.google.firebase:firebase-core:12.0.1'
    implementation 'com.google.firebase:firebase-messaging:12.0.1'
}

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

build.gradle (Project: agenda):

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.1.3'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
        classpath 'com.google.gms:google-services:3.2.0'
    }
}

allprojects {
    repositories {
        jcenter()
        maven { url "https://maven.google.com" }
    }
}