Undefined index

Undefined index: addthis_asynchronous_loading in wp-content/plugins/addthis/includes/addthis_addjs.php on line 168.

This is a fix for it:

function addWidgetToJs(){
        ...
        $addthis_asynchronous_loading = isset($addthis_settings_options['addthis_asynchronous_loading']) ? $addthis_settings_options['addthis_asynchronous_loading'] : null;
        if($addthis_asynchronous_loading) {
        ...
    }

https://wordpress.org/plugins/addthis/