title: How do I update the photoUrl ? photoUrl : null
body: <p>I want to update photoUrl but from my code I can upload images to storage but in my document photoUrl : Null</p>
  <p><strong>updatePhoto.dart</strong></p>
  <pre><code>void _savePhotoToFirebase() async {
      String userID = Utils.getRandomString(8) + Random().nextInt(500).toString();
      String postPhotoUrl;
      FirebaseUser currentUser = await _auth.currentUser();
      var userUpdateInfo = UserUpdateInfo();
      userUpdateInfo.photoUrl = postPhotoUrl;
      if (_postPhotoFile != null) {
        postPhotoUrl = await CSStorage.uploadPhotoUrl(
            userID: userID, postPhotoFile: _postPhotoFile);
      }
      FBCloudStore.sendPhotoToFirebase(
          userID, postPhotoUrl ?? 'NONE', currentUser);
    }
  </code></pre>
  <p><strong>CSStorage.dart</strong></p>
  <pre><code>    static Future&lt;String&gt; uploadPhotoUrl({
          @required String userID,
          @required File postPhotoFile,
          }) async {
          try {
            String fileName = 'users/$userID/photoUrl';
            StorageReference reference = FirebaseStorage.instance.ref().child(fileName);
            StorageUploadTask uploadTask = reference.putFile(postPhotoFile);
            StorageTaskSnapshot storageTaskSnapshot = await uploadTask.onComplete;
            String postPhotoUrl = await storageTaskSnapshot.ref.getDownloadURL();
            return postPhotoUrl;
          }catch(e) {
            return null;
          }
        }
  </code></pre>
  <p><strong>FBCloudStore.dart</strong></p>
  <pre><code>static Future&lt;void&gt; sendPhotoToFirebase(
        String userID, String postPhotoUrl, FirebaseUser firebaseUser) async {
      Firestore.instance.collection('users').document(userID).setData({
        'userID': userID,
        'displayName': firebaseUser.displayName,
        'photoUrl': firebaseUser.photoUrl,
      });
    }
  </code></pre>