Sounds like $my_theme_option has no ‘custom_sidebars’ index, if you’ve remove it, it does not exist anymore.
just add a if isset() around it, just like you did at around the for loop
Also you don’t need to check for functions like register_sidebar to exist if they are older then 2 years.
EDIT: DONT USE GLOBALS! There are functions for that e.g get_option()
// CUSTOM SIDEBARS
global $my_theme_option;
if(isset( $my_theme_option[ 'custom_sidebars' ] )){
$my_theme_custom_sidebars = $my_theme_option[ 'custom_sidebars' ];
if ( sizeof( $my_theme_custom_sidebars ) > 0 ) {
foreach ( (array)$my_theme_custom_sidebars as $sidebar ) {
register_sidebar( array(
'name' => $sidebar,
'id' => 'sidebar-' . $sidebar,
'before_widget' => '<div id="%1$s" class="widget %2$s">',
'after_widget' => '</div> <!-- end of widget -->',
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>'
));
}
}
}