Not overwrite Main Index by Page.

config.yaml

callbacks:
    cms_save_filter.page: >
        sub {
            my ( $cb, $app ) = @_;
            if ( ( $app->param( 'category_ids' ) eq '-1' ) &&
                ( $app->param( 'basename' ) eq 'index' ) ) {
                return $cb->error( $app->translate( 'File [_1] exists; could not overwrite.', 
                    $app->translate( 'Main Index' ) ) );
            }
            1;
        }