fix(RecycleScroller): gridItems is undefined when scrollToItem, fix #773

[crs38c28]

Description

Fix RecycleScroller scrollToItem method when itemSize is not null.  
Scroll position is wrong because gridItems is undefined.

Reproduction
Demo site

1. unchecked 'variable height'
2. click 'Scroll To:' button
3. list scroll to 'item 0' (not 'item 100')

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[mjclyde]

I just noticed this issue, made the same fix for it, then noticed your PR. 👍🏻 Nice catch.