flutterのプッシュ通知(fcm)で、releaseビルドの時だけ受信時にエラーになる

Question

flutterで、fcmを利用したプッシュ通知を実装しようとしています。
debugビルドの時はうまくプッシュ通知を受信できるのですが、
releaseビルドの時に、プッシュ通知を受信した瞬間にエラーとなって、アプリが落ちてしまいます。

import 'dart:async';
import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_messaging/firebase_messaging.dart';
import 'package:flutter/material.dart';
import 'package:flutter_local_notifications/flutter_local_notifications.dart';

Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  // If you're going to use other Firebase services in the background, such as Firestore,
  // make sure you call `initializeApp` before using other Firebase services.
  await Firebase.initializeApp();
  print('Handling a background message ${message.messageId}');
}

const AndroidNotificationChannel channel = AndroidNotificationChannel(
  'high_importance_channel', // id
  'High Importance Notifications', // title
  'This channel is used for important notifications.', // description
  importance: Importance.high,
);

final FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin =
    FlutterLocalNotificationsPlugin();

Future<void> firebaseInit() async {
  print("firebaseInit");

  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();

  // Set the background messaging handler early on,
  // as a named top-level function
  FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

  /// Create an Android Notification Channel.
  ///
  /// We use this channel in the `AndroidManifest.xml` file to override the
  /// default FCM channel to enable heads up notifications.
  await flutterLocalNotificationsPlugin
      .resolvePlatformSpecificImplementation<
          AndroidFlutterLocalNotificationsPlugin>()
      ?.createNotificationChannel(channel);

  /// Update the iOS foreground notification presentation options to allow
  /// heads up notifications.
  await FirebaseMessaging.instance.setForegroundNotificationPresentationOptions(
    alert: true,
    badge: true,
    sound: true,
  );

  FirebaseMessaging.instance.getInitialMessage().then((RemoteMessage? message) {
    //print("message : " + message.toString());
  });

  FirebaseMessaging.onMessage.listen((RemoteMessage message) {
    final notification = message.notification;
    final android = message.notification?.android;
    //print("notification : " + notification.toString());

    if (notification != null && android != null) {
      flutterLocalNotificationsPlugin.show(
          notification.hashCode,
          notification.title,
          notification.body,
          NotificationDetails(
            android: AndroidNotificationDetails(
              channel.id,
              channel.name,
              channel.description,
              // ignore: flutter_style_todos
              // TODO add a proper drawable resource to android, for now using
              //      one that already exists in example app.
              icon: 'launch_background',
            ),
          ));
    }
  });

  //runApp(Permissions());
  /*
  NotificationSettings settings =
  await FirebaseMessaging.instance.requestPermission(
    announcement: true,
    carPlay: true,
    criticalAlert: true,
  );

   */
}

flavorで、devとprodで分けていますが、
devもprodも、debugモードのみプッシュ通知を正常に受信でき、releaseモードの時にエラーとなってしまいます。

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
        localProperties.load(reader)
    }
}

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
apply plugin: 'com.google.gms.google-services'

android {
    compileSdkVersion 30

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }

    lintOptions {
        disable 'InvalidPackage'
        checkReleaseBuilds false // issue https://github.com/flutter/flutter/issues/22397
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.example.test"
        minSdkVersion 23
        targetSdkVersion 30
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

    // SigningConfigs
    apply from: 'signingConfigs/debug.gradle', to: android
    apply from: 'signingConfigs/release.gradle', to: android

    buildTypes {
        release {
            signingConfig signingConfigs.release
        }
        debug {
            debuggable true
            signingConfig signingConfigs.debug
        }
    }


    flavorDimensions "app"
    productFlavors {
        dev {
            dimension "app"
            resValue "string", "app_name", "DEV_test-app"
            applicationIdSuffix ".dev"
        }
        prod {
            dimension "app"
        }
    }

}

flutter {
    source '../..'
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation "com.google.firebase:firebase-messaging:21.0.1"
}

実機デバックすれば原因がわかりそうなものなのですが、
実機デバックの際はdebugモードになってしまい（releaseモードにはできない？）、
正常に成功してしまいます。