JSON 对象存在，但当选择属性时，在离子 2 上返回未定义

我对 ionic 2 有一个非常问题，我正在获取一个 JSON 对象，我可以使用代码<span>{{ user|json }}</span>检查对象数据是否存在，但是当我使用代码时<span>{{ user.id }}</span>会显示异常，并显示消息"无法读取 null 的属性 ID"。this.userService.get( this.user_id )方法用于另一个部分，工作正常，但在本节中没有，你能帮我吗？

这是我的代码：

    <!-- my profile.html view -->
    <ion-row>
              <ion-col col-4>
                Tipo de socio
              </ion-col>
              <ion-col col-8>
                <span>{{ user.id }}</span><!-- cannot read id property of a null -->
              </ion-col>
            </ion-row>
            <ion-row>
              <ion-col col-4>
                Tipo de socio
              </ion-col>
              <ion-col col-8>
                <span>{{ user|json }}</span><!-- the data is returned -->
              </ion-col>
            </ion-row>
    // My profile.ts ( where I'm loading the view and calling the user services )
    import { User, UserService } from '../../app/models/user';
    @Component({
      selector: 'page-profile',
      templateUrl: 'profile.html'
    })
    export class ProfilePage {
      title = 'My Profile';
      user: User = null;
      private user_id: number = 0;
      constructor(
        public navCtrl: NavController,
        public navParams: NavParams,
        public alertCtrl: AlertController,
        public modalCtrl: ModalController,
        public loadingCtrl: LoadingController,
        public userService: UserService) {
        this.user_id = this.navParams.get('user_id'); // this value is working fine!
        if ( this.user_id > 0 ) {
          this.userService.get( this.user_id ).then( user => this.user = user );
        }
      }
      ionViewDidLoad() { }
    }
    // My user.ts ( where I've my services )
    @Injectable()
    export class User {
      id: number;
      name: string;
      email: string;
      ic: number;
      birth_date: string;
      home_phone: number;
      phone: number;
      is_active: boolean;
      is_admin: boolean;
      from_social_account: boolean;
      is_superuser: boolean;
      status: boolean;
      created_at: any;
      updated_at: any;
      groups: any;
      permissions: any;
      addresses: any;
      all_permissions: any;
      auth_token: string;
      is_customer: boolean;
      is_partner: boolean;
      last_login: string;
      profile: any;
      requests: any;
      skills: any;
      social_providers: any;
      wallets: any;
      errors: any = null;
      birth_date_no_formatted: string;
      main_address: any;
    }
    @Injectable()
    export class UserService {
      private headers: Headers = new Headers(Constant.DEFAULT_HEADERS);
      private url: string = `${Constant.API_HOST}${Constant.API_PREFIX}${Constant.API_V1}`;
      constructor(
        private http: Http) { }
      get( _id: number ): Promise<User> {
        let url = `${this.url}users/${_id}/`;
        return this.http.get(url, { headers: this.headers })
          .toPromise().then( response => response.json() as User ).catch(this.handleError);
      }
      
      private handleError(error: any): any {
        console.log( JSON.stringify( error ) );
        return {errors: StaticMethods.formatSubmitErrors( error.json() ) };
      }
    }

{{ user?.id }}