First thing to do is deactivate ALL non weaver plugins and see if the issue goes away in case one in interacting.
If not, you can do a full reset of weaver stuff in case something became corrupt.
- Save your weaver settings and download a copy for good measure.
- Switch to a different theme
- Deactivate and DELETE all weaver theme and plugins.
- Reinstall Xtreme latest version from WordPress, activate and see if the customizer now works.
- If it does reinstall all weaver plugins from WordPress, and if you have Plus, download a fresh copy from your weaver account and reinstall / activate