Reset or Clean button for ion-select and ion-option

Ionic Framework ionic-v3
1

Basically, how can i clean a ionic select with a button? I’ve try doing this.escolhido = null but get me this error:

“ERROR Error: Query: First argument passed to startAt(), endAt(), or equalTo() cannot be an object.”

I understand the error and why its happening but how can i fix it? Here is my codes:

admin.html

<ion-header>
  <ion-navbar color="amarelo">
    
    <ion-buttons left>
      <button ion-button icon-only menuToggle>
        <ion-icon name="menu"></ion-icon>
      </button>
    </ion-buttons>

    <ion-title >
        Administrador
    </ion-title>

  </ion-navbar>
</ion-header>

<ion-content padding>
  <ion-grid>
    <ion-row>
        <ion-label>Categorias</ion-label>
          <ion-select (ionChange)="onContextChange($event)" [(ngModel)]="escolhido"
          cancelText="Cancelar" okText="Ok">
          <ion-option *ngFor="let gene of genesList$ | async"
          detail-push>{{gene.nome}}</ion-option>
          </ion-select>
        <button ion-button (click)="limpaFiltro()" color="amarelo">
          Limpar
        </button>
    </ion-row>
    
    <ion-row>
        <ion-list>
            <ion-list-header class="estilo">
              Producoes
            </ion-list-header>
            <ng-container *ngFor="let prod of prodsList$ | async | slice:0:show; let i=index">
                <ion-item 
                detail-push navPush="ViewProdPage"[navParams]="{prod:prod}">
                {{prod.titulo}}
              </ion-item>
            </ng-container>  
        </ion-list>
    </ion-row>
    <ion-row>
      <button full ion-button (click)="loadMore()" color="amarelo">
        Carregar mais
      </button>
    </ion-row>
  </ion-grid>
</ion-content>

<ion-footer>
    <ion-fab bottom right>
        <button color="amarelo" ion-fab navPush="AddProdPage"><ion-icon name="add"></ion-icon></button>
      </ion-fab>
</ion-footer>

admin.ts

import { Component } from '@angular/core';
import { IonicPage, NavController, NavParams } from 'ionic-angular';
import { Producao } from '../../models/producao';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { ProducoesProvider } from '../../providers/producoes/producoes'
import { Genero } from '../../models/genero';

/**
 * Generated class for the AdminPage page.
 *
 * See https://ionicframework.com/docs/components/#navigation for more info on
 * Ionic pages and navigation.
 */

@IonicPage()
@Component({
  selector: 'page-admin',
  templateUrl: 'admin.html',
})
export class AdminPage {

  public show: number;
  escolhido: Object;
  prodsList$: Observable<Producao[]>;
  genesList$: Observable<Genero[]>;


  constructor(public navCtrl: NavController, public navParams: NavParams, 
    private producoesProvider: ProducoesProvider) {
      this.show = 5;

      this.prodsList$ = this.producoesProvider.getProdsList().valueChanges();
  }

  ionViewWillLoad(){
    this.genesList$ = this.producoesProvider.getGenesList().valueChanges();
  }


  limpaFiltro(){
    this.escolhido = null;
  }

  loadMore() {
    this.show += 5;
  }

  onContextChange(ctxt: string): void {
    this.prodsList$ = this.producoesProvider.assembleGeneroFilteredList(ctxt).valueChanges();

  }

}