Consider dropping or fixing saveScrollPosition

[henriksteen]

Changing line 437 from
initialScrollIndex={saveScrollPosition ? selectedIndex : -1}
to
initialScrollIndex={saveScrollPosition ? selectedIndex : 0}
makes it possible to disable saveScrollPosition and avoid the -1 warnings.

As it happens saveScrollPosition doesn't work reliably on the latest versions of react anyway, often not rendering the full list, so simply having the option to directly override initialScrollIndex to 0 would be better.