Crash on update in CollectionViewDataSource #20

I'm experimenting with the code in a slightly different context (so there may be no bug in the example app), but I believe there's a lurking bug in the implementation of

processUpdates(updates: [DataProviderUpdate<Data.Object>]?)

if an update is received for an object (say via the FetchedResultsDataProvider invoking it's delegate - DataProviderDelegate.dataProviderDidUpdate(updates: [DataProviderUpdate<Object>]?)) for an object which is known and in the model, but has become off-screen, then inside the batch update's .Update(let indexPath, let object): clause, then the line

guard let cell = self.collectionView.cellForItemAtIndexPath(indexPath) as? Cell else { fatalError("wrong cell type") }

will fail because self.collectionView.cellForItemAtIndexPath(indexPath) can return nil for cells which are out of range, or simply because they're offscreen. If there's no cell, it's likely the right approach is to check for a valid cell before proceeding and retain the type check on the cell type for safety (i.e.. split out the nil check from the type check)... something like

                case .Update(let indexPath, let object):
                    // Note: cell can be offscreen and therefore nil
                    if let c = self.collectionView.cellForItemAtIndexPath(indexPath) {
                        guard let cell = c as? Cell else { fatalError("wrong cell type") }
                        cell.configureForObject(object)
                    }