Prevents SC.CollectionView.prototype.itemViewForContentIndex() from returning item views for invalid indexes. It's easy for loops and other code to accidentally retrieve an index not within the collection, which would have previously returned an item view not backed by content and with other invalid attributes. Instead, the method will return null for invalid indexes and in debug mode only will present a warning to help the developer track down the problem. Includes unit tests.
Fixes exception when hitting the return key on editable collection view with no selection. Refactored the method to prevent attempting to edit invalid item views. Includes unit test.