TypeError: Cannot read property ‘uid’ of null

gistfile1.txt

import { Component } from '@angular/core';
import { NavController, ToastController } from 'ionic-angular';
import { PostFbProvider } from '../../providers/post-fb/post-fb';
import firebase from 'firebase';
import { AddPostPage } from '../add-post/add-post';

@Component({
  selector: 'page-home',
  templateUrl: 'home.html'
})
export class HomePage {

  public post;
  public uid:any;

  constructor(public navCtrl: NavController, public toastCtrl: ToastController, private postFB: PostFbProvider) {
    this.uid = firebase.auth().currentUser.uid;
  }

  ionViewDidLoad(){
    this.postFB.all().on('value', snapshot => {
      this.post = [];
      snapshot.forEach(snap => {
          this.post.push({
            id: snap.key,
            uid: snap.val().uid,
            title: snap.val().title,
            body: snap.val().body
          });
          return false;
      })
    })
  }

  newPost(){
    this.navCtrl.push(AddPostPage);
  }
}