I tried add a simple text box to 404 page Not Found Content widget but the theme customizer stop working, the widget menu missing and the live preview don’t load. I tried with only a search widget but i got the same result. The text and the search box appear the 404 page but the customizer did not work anymore. If i delete the widget from 404 Page Not Found Content area, the theme customizer work perfectly. I don’t use child theme and i tried reinstall the wordpress with no plugins, but the result is same.