It seems that something in your project is generating output before the Plugins initialization. You can check the generated HTML code of the page where you are getting the error; the code above the reported error is generated before the Plugins initialization and stops the plugin from initializing correctly.
Sorry for the late reply but we didn’t receive the notification.
We can’t add that option for the WordPress back-end but we are going to consider the implementation for the front-end search in the PRO version of the plugin.
it seems that you’re running your project on an outdated PHP version (< 5.4) which doesn’t support the short syntax for arrays definition. This plugin requires PHP >= 5.4.