I found temporary workaround & code fixes for this. (As of 14 May 2019)
WORKAROUND:
If you click on the 'Code' tab first, you can then click 'Settings' and it will load.
CODE FIX:
For a more functional fix, you can edit the plugin on your server. Looks like the builder.js file in the MB Builder plugin is attempting to instantiate a new Clipboard object using Clipboard instead of ClipboardJS -- If you wish to fix it for now, you can edit the builder.js file as follows...
Go to this page in your WP plugin editor:
http://yourdomain.com/wp-admin/plugin-editor.php?file=meta-box-builder%2Fassets%2Fjs%2Fbuilder.js&plugin=meta-box-builder%2Fmeta-box-builder.php
On line 550 you should see:
clipboard = new Clipboard( '.mbb-button--copy', {
Change new Clipboard to new ClipboardJS
So the full new line 550 would read:
clipboard = new ClipboardJS( '.mbb-button--copy', {
Save the change, then go back to the Field Group editor and try it out. (You might need to close your old browser tab/window and re-open the page, and/or refresh your browser 5 million times for it to load the the updated builder.js file)
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