ItemsViewController null checks

[reid-kirkpatrick]

Description of Change

In the ItemsViewController multiple methods (i.e. UICollectionViewController.GetCell() which calls ItemsViewController.UpdateTemplatedCell() and UICollectionViewController.GetItemsCount() and UICollectionViewController.NumberOfSections() which both call ItemsViewController.CheckForEmptySource()) can be called by iOS after ItemsViewController.Dispose() has been called and _measurementCells has been set to null. There are already null checks for _measurementCells in certain places, but not all. This should cover the rest of the cases where it can be null.

Issues Resolved

• fixes [Bug] Go backwards crash the app while CollectionView scrolling #15637

API Changes

None

Platforms Affected

• iOS

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

I was able to reproduce the crash in the attached sample from #15637. I then created a local nuget package from the 5.0.0 branch and confirmed the cause. After making these changes I created another nuget package, updated the repro sample and confirmed the crash no longer happened.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[jtorvald]

@reid-kirkpatrick I just created a patch and saw that you already fixed it last week. Thanks!

@jfversluis can this one please be merged and released on short notice?
It's a very annoying issue with the CollectionView and makes it almost unusable.

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Thanks for this @reid-kirkpatrick ! Appreciate you looking into this and opening a PR!

Don't think this is a shocking change so I don't see any issues merging this. Releasing on short notice I can't promise, but it will be part of the next service release when merged.

[jtorvald]

Thanks @jfversluis and @reid-kirkpatrick !

[jfversluis]

Confirmed here that this build works.

Thanks again @reid-kirkpatrick !

[beeradmoore]

@jfversluis , do we have any ETA of Xamarin.Forms 5.0 SR 14 where this fix will be included?

[reid-kirkpatrick]

@jfversluis any update here? We just released a new version of our app that added some uses of controls affected by this and our crash rate is a little troubling.

[jfversluis]

Working on a new SR, should be soon. Hoping next week, but no solid promises at this point.

[ardentra]

@jfversluis 🙏 thank you!

[oridotaoyebode]

@jfversluis any news on the SR for this week? Pretty please.

[jtorvald]

@oridotaoyebode it has been released yesterday