backButtonSubscription not working

Ionic Framework
1

hi,
help me in this.

import { Platform } from '@ionic/angular';
  constructor(
    private platform: Platform,
  ){
    this.backButtonSubscription = this.platform.backButton.subscribe(() => {
      alert('lll');
     });
  }


ngOnDestroy()
 {
    this.backButtonSubscription.unsubscribe();
   }