Customizer load issue

If the Customizer doesn't load, check the console (press F12 in Chrome to bring it up), see if it shows "/ invalid data" error. If so, add this snippet to the custom-functions.php file:

function custom_fix_customizer_error() {
  $mods = get_theme_mods();
  $mods = stripslashes_deep( $mods );
  update_option( "theme_mods_" . get_option( 'stylesheet' ), $mods );
}
add_action( 'get_header', 'custom_fix_customizer_error' );

Load the homepage once, then remove that snippet.