All Elements Disappearing from Elements Sidebar
The Elements Sidebar is found on Page Builder Sandwich that shows up upon hovering on the left side of the screen. This is where you get the elements to drag on your content area.
An empty Elements Sidebar is usually caused by external jQuery libraries being enqueued. It's likely that theme is causing this. This can cause problems not only to Page Builder Sandwich, but on other plugins as well.
Sometimes, modified themes start including their own copy of jQuery, and override the one supplied by WordPress. Because of this, Sandwich encounters an error since a few libraries it will use get overwritten and cannot be found anymore.
WordPress already has it's own copy of jQuery that many plugins rely on and it's generally not a good idea to replace it with another copy.
To fix this, find the jQuery enqueue in your code that isn't from WordPress Core, and enqueue the one shipped with WordPress.
If you would like to know why loading another jQuery library aside from the one shipped with WordPress is not a good idea, here's a great article about it that you can check out: https://pippinsplugins.com/why-loading-your-own-jquery-is-irresponsible/