Welcome to the BB forums!
BB supports the WP Revisions system. Can you try reverting to an earlier version of your site, prior to adding the code, and see if that fixes it? You can access it via the native WP editor.
Is there a best practice for popping up a modal email signup from a button?
Not really sure on that regard. Maybe ask the FB Group and/or Slack Channel for the same. We’ve a lot of active users there.
If I misunderstood you in that you simply wanted to use a modal popup, there isn’t a BB module for that but there should be a lot of 3rd party plugins that offer the same. I’ve tried Popup Maker before and it works fine.
This has also been requested before so if you’ve a minute, feel free to vote for it on the link below.