It’s a known issue, will be fixed within the next release. It’s only a undefined variable notice coming from the back-end, it should not affect the site at all.
If you want to fix it now, you can do it manually, by opening the \wp-content\plugins\ajax-search-pro\includes\classes\actions\class-asp-indextable.php file and replacing line 25
$it_options = get_option('asp_it_options');
$it_options = wd_asp()->o['asp_it_options'];
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