title: DownloadURL return null in below code
body: <p>When I change the file its successfully updated in firebase storage but I when I try to get downloadURL its show null.</p>
  
  <pre><code>fileChange(event: any) {
      const imageFolder: string = this.employee.id;
      const fileList: FileList = event.target.files;
      const file: File = fileList[0];
      const storageRef = firebase.storage().ref().child(`${imageFolder}/profile.jpg`).put(file);
      console.log(storageRef.snapshot.downloadURL);
  }
  </code></pre>