code: func InitFCMClient(key string) (*fcm.Client, error) {
  	var err error
  
  	if key == "" {
  		return nil, errors.New("Missing Android API Key")
  	}
  
  	if key != PushConf.Android.APIKey {
  		return fcm.NewClient(key)
  	}
  
  	if FCMClient == nil {
  		FCMClient, err = fcm.NewClient(key)
  		return FCMClient, err
  	}
  
  	return FCMClient, nil
  }
docs: // InitFCMClient use for initialize FCM Client.