Summary:The bug is caused by a weird race condition. What happens is that when calling `UIRefreshControl#endRefreshing` the `UIScrollView` delegate `scrollViewDidScroll` function is called synchronously and then `dockClosestSectionHeader` crashes because the sticky header indexes are updated but not the contentView children.
I fixed it by adding an updating property on `RCTRefreshControl` and setting it before calling `endRefreshing` so we can know not to call `dockClosestSectionHeader` at that moment.
Tested with both `RefreshControl` and `onRefreshStart` prop.
I reproduced the bug by replacing ListViewExample.js in UIExplorer with https://gist.github.com/janicduplessis/05fc58e852f3e80e51b9Fixes#5440
cc nicklockwood
Closes https://github.com/facebook/react-native/pull/5445
Differential Revision: D2953984
Pulled By: nicklockwood
fb-gh-sync-id: c17a6a75ab31ef54d478706ed17a8115a11d734e
shipit-source-id: c17a6a75ab31ef54d478706ed17a8115a11d734e
671b975d92