em_selectize is not a function

Hello.

I’m trying to fix a bug on the website above that prevents a search bar located here:
https://greenevo.gov.pl/pl/zielone-technologie/
from working. Inspecting the page shows this:

jQuery.Deferred exception: container.find(...).em_selectize is not a function em_setup_selectize@https://greenevo.gov.pl/wp-content/plugins/events-manager/includes/js/events-manager.js?ver=7.2.2.1:1887:94
em_setup_ui_elements@https://greenevo.gov.pl/wp-content/plugins/events-manager/includes/js/events-manager.js?ver=7.2.2.1:353:20
@https://greenevo.gov.pl/wp-content/plugins/events-manager/includes/js/events-manager.js?ver=7.2.2.1:340:22
e@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:27028
Deferred/then/l/</t<@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:27330
setTimeout handler*Deferred/then/l/<@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:27594
c@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:25304
fireWith@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:26053
fire@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:26089
c@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:25304
fireWith@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:26053
ready@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:29112
P@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:28884
EventListener.handleEvent*@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:29264
@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:220
@https://greenevo.gov.pl/wp-includes/js/jquery/jquery.min.js?ver=3.7.1:2:225
 undefined

Uncaught TypeError: container.find(...).em_selectize is not a function
    em_setup_selectize selectize.js:135
    em_setup_ui_elements index.js:353
    <anonymous> index.js:340
    jQuery 13
        e
        t
        setTimeout handler*Deferred/then/l/<
        c
        fireWith
        fire
        c
        fireWith
        ready
        P
        EventListener.handleEvent*
        <anonymous>
        <anonymous>
selectize.js:135:94

Looking at the files of the currently used theme, I see em_selectize.js in the js folder. Not sure why I get this error then.

The page I need help with: [log in to see the link]