友盟客户端日志显示收到日志，却没弹出通知栏提示

fun init(context: Context, appKey: String, messageSecret: String) {
    Log.i(TAG, "初始化")
    UMConfigure.init(context, appKey, CHANNEL, UMConfigure.DEVICE_TYPE_PHONE, messageSecret)
    val pushAgent = PushAgent.getInstance(context)
    //设置App处于前台时显示通知
    pushAgent.notificationOnForeground = true
    //设置最多显示通知的条数
    pushAgent.displayNotificationNumber = 0
    //检查集成配置文件
    pushAgent.isPushCheck = true
    //注册推送
    pushAgent.register(object : UPushRegisterCallback {
        override fun onSuccess(deviceToken: String?) {
            Log.i(TAG, "注册成功 deviceToken:$deviceToken")
        }

        override fun onFailure(errCode: String?, errDesc: String?) {
            Log.e(TAG, "注册失败 code:$errCode desc:$errDesc")
        }
    })

    val msgHandler: UmengMessageHandler = object : UmengMessageHandler() {
        //处理通知栏消息
        override fun dealWithNotificationMessage(context: Context, msg: UMessage) {
            super.dealWithNotificationMessage(context, msg)
            Log.d(TAG, "接收到推送消息")
        }

        //自定义通知样式，此方法可以修改通知样式等
        override fun getNotification(context: Context, msg: UMessage): Notification {
            return super.getNotification(context, msg)
        }

        //处理透传消息
        override fun dealWithCustomMessage(context: Context, msg: UMessage) {}
    }
    pushAgent.messageHandler = msgHandler
    val notificationClickHandler: UmengNotificationClickHandler =
        object : UmengNotificationClickHandler() {
            override fun dealWithCustomAction(context: Context, msg: UMessage) {}
            override fun openActivity(context: Context, msg: UMessage) {
                //todo 打开指定的页面
            }
            override fun launchApp(context: Context, msg: UMessage) {}
            override fun dismissNotification(context: Context, msg: UMessage) {}
        }
    pushAgent.notificationClickHandler = notificationClickHandler
}

上述代码为初始化相关，当我注释掉

//pushAgent.messageHandler = msgHandler

不设置messageHandler就又能正常弹出通知，这是为啥？