Uncaught TypeError: b.selectize is not a function

There is an issue when submitting a question (https://thriveworks.com/community/questions/ask-a-question/). The following is displayed in console.

common.js:846 Uncaught TypeError: b.selectize is not a function
    at Object.AnsPress.tagElements (common.js:846)
    at HTMLInputElement.<anonymous> (common.js:850)
    at Function.each (jquery-2.1.4.min.js?ver=5.2.4:2)
    at n.fn.init.each (jquery-2.1.4.min.js?ver=5.2.4:2)
    at HTMLDocument.<anonymous> (common.js:849)
    at j (jquery-2.1.4.min.js?ver=5.2.4:2)
    at Object.fireWith [as resolveWith] (jquery-2.1.4.min.js?ver=5.2.4:2)
    at Function.ready (jquery-2.1.4.min.js?ver=5.2.4:2)
    at HTMLDocument.I (jquery-2.1.4.min.js?ver=5.2.4:2)
admin-ajax.php:1 Failed to load resource: the server responded with a status of 500 (Internal Server Error)