AdvancedCollectionView: Exception thrown in ItemOnPropertyChanged when live shaping enabled but no filter set

[TheShihan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  <!-- Please search GitHub for a similar issue or PR before submitting -->
[ ] Feature request <!-- Please file a UserVoice request and include the link below https://wpdev.uservoice.com/forums/110705-universal-windows-platform/category/193402-uwp-community-toolkit -->
[ ] Sample app request
[ ] Documentation issue or request
[ ] Question of Support request => Please do not submit support request here, instead see https://github.com/Microsoft/UWPCommunityToolkit/blob/master/contributing.md#question

Current behavior

I'm using an AdvancedCollectionView bound to a ListView to display items sorted by a DateTime property. I noticed that an exception occured in the OnPropertyChanged of the bound items when live shaping was enabled (true passed as second parameter to AdvancedViewCollection). After further analysis I saw that the NullReferenceException was thrown inside method ItemOnPropertyChanged of AdvancedCollectionView. This line throws the Exception when no filter has been set:
var filterResult = _filter(item);
So I changed my setup of the AVC from:

                    _acv = new AdvancedCollectionView(_timeEntries, true);
                    _acv.SortDescriptions.Add(new SortDescription("DateAndTime", SortDirection.Ascending));

to the following:

                    _acv = new AdvancedCollectionView(_timeEntries, true);
                    _acv.Filter = te => !string.IsNullOrWhiteSpace(((TimeEntry)te).Title);
                    _acv.SortDescriptions.Add(new SortDescription("DateAndTime", SortDirection.Ascending));

I added a dummy filter, that filters out items without a title (which never occurs anyway), just so that I don't run into the NullReferenceException.

Expected behavior

Live Shaping should work without a filter set.

Minimal reproduction of the problem with instructions

Initialise an AVC with live shaping enabled, don't setup a filter
AVC should consume an ObserverableCollection and bind the AVC to the ItemSource of a ListView.
Make sure the items of the collection implement INotifyPropertyChanged and you fire an event for property changes. Make use of the properties inside the ListView (data template). Edit a property to make the event fire. Exception should occur when OnPropertyChanged fires.

Environment

Nuget Package(s): 
- Microsoft.NETCore.UniversalWindowsPlatform

Package Version(s): 
- Microsoft.NETCore.UniversalWindowsPlatform 6.0.4

Windows 10 Build Number:
- [ ] Anniversary Update (14393) 
- [ ] Creators Update (15063)
- [ X] Fall Creators Update (16299)
- [ ] Insider Build (xxxxx)

App min and target version:
- [ X] Anniversary Update (14393) 
- [ ] Creators Update (15063)
- [ X] Fall Creators Update (16299)
- [ ] Insider Build (xxxxx)

Device form factor:
- [X] Desktop
- [ ] Mobile
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 15.3
- [X] 2017 15.4

[nmetulev]

@Shayon, any thoughts on this?

[Shayon]

I think I see the issue. Looks like I missed this. 😞 I'll look into adding a test and fixing the functionality.

For now the workaround identified by @TheShihan will fix this issue. i.e. Adding a dummy filter like: _acv.Filter = x => true; when live shaping is enabled.

[skendrot]

Merged in #1688