Firebase push fail

Ionic Framework ionic-v3
1

Hello helpers, I am pushing data into Firebase but I am having this error. What have I missed out or done wrong?
Earlier I am able to push data into it but not on this page.
Much appreciation.
I would like to push this value into Firebase for later retriever.

Error message

ERROR Error: Firebase.push failed: first argument contains undefined in property 'all-item.anArray'.

My html code

<ion-content>
    <ion-item>
      <ion-label>Number of item</ion-label>
    <ion-input type="number" [(ngModel)]="item" ></ion-input>
  </ion-item>

  <button ion-button (click)="Add(item)">Create Field</button>

  <button ion-button (click)="delete(item)">Remove Field</button>
  
    <ion-item *ngFor="let att of anArray; let idx = index">
        <ion-label color="primary" floating>{{att.label}}{{idx+1}}</ion-label>
        <ion-input type="text" [(ngModel)]="anArray[idx].value"></ion-input>
    </ion-item>

My ts code

export class HomePage {

  anArray: any = [];

  item: any;
  btn: any
  data: any;

  allItem = {} as allItem;

  allItemRef$: FirebaseListObservable<allItem[]>

  constructor(public navCtrl: NavController, public navParams: NavParams, private database: AngularFireDatabase) {

    this.allItemRef$ = this.database.list('all-item');

    console.log('this.anArray', this.anArray);
    this.btn = false;
    this.data = false;
  }

  Add(key) {
    console.log('key', key)
    for (let i = 0; i < key; i++) {
      this.anArray.push({ label: 'item', value: '' });
    }
    console.log('show current', this.anArray);
    this.btn = true;
  }


  delete(key) {
    console.log('key', key)
    for (let i = 0; i < key; i++) {
      this.anArray.pop({ label: 'item', value: '' });
    }
    console.log('show current', this.anArray);
    this.btn = true;
  }

  submit(allItem: allItem) {
    this.allItemRef$.push({
      anArray: this.allItem.anArray,
    });

    this.allItem = {} as allItem;

    this.navCtrl.push('NextPage');
  }

My model class

export interface allItem {
    anArray: string;
}

Thank you for all help.