highlightAtIndex method
Highlight the item at the given original index (index in the items list)
Implementation
void highlightAtIndex(int index) {
if ((_state?._isExpanded == false)) {
_state?._openFromController();
}
/// Highlight the item at `index` where `index` represents
/// the index in the original `items` list passed to `SDropdown`.
/// If the overlay is not open, this will open it and then highlight the item.
_state?._setHighlightAtIndex(index);
}