TypeError: $.ui and $element.dialog is undefined I have been handed a site to work on. It's on 8.5.1. The site was working. However, I cannot make changes on certain configs. For example, the dialog boxes for the views do not open anymore. I keep getting the following js error under the console. 

I do not want to revert the database back to a working condition since the client would lose their data.

Update: The dialog box does work very very random.
Thank you everyone for helping. 
I informed of the senior development of your suggestions. He was able to find the issue. Font Awesome module was installed incorrectly, he also updated to the latest core. We've uninstall Font Awesome and migrated to using Font Awesome CDN.