Crash on fast header tap

[ignatovSA]

Multiply tapping on header leads to crash:

2016-07-24 21:07:40.052 ExpandableColumnLayout_Example[84459:20481756] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'attempt to scroll to invalid index path: <NSIndexPath: 0xc000000000000116> {length = 2, path = 1 - 0}'

Exception comes from
SectionExpander.swift -> toggleExpansionForSectionAtIndex -> UIView.animateWithDuration -> collectionView.scrollToItemAtIndexPath

[johntvolk]

@ignatovSA, thanks for the report.

I have a fix for this in master now. I will try to get a release out to cocoapods soon. It's been a while since I last released this project so hopefully there won't be too many things I need to fix up for release.