Cant activate sidebar for my theme

Hey,

I’m using multiple themes within my wordpress instance and I’m handling the themes with the “jonradio Multiple Themes”-plugin. However, in one of that themes I cant get enabled the sidebar. In functions.php there is

function twentythirteen_widgets_init() {
	register_sidebar( array(
		'name'          => __( 'Main Widget Area', 'twentythirteen' ),
		'id'            => 'sidebar-1',
		'description'   => __( 'Appears in the footer section of the site.', 'twentythirteen' ),
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h3 class="widget-title">',
		'after_title'   => '</h3>',
	) );

	register_sidebar( array(
		'name'          => __( 'Secondary Widget Area', 'twentythirteen' ),
		'id'            => 'sidebar-2',
		'description'   => __( 'Appears on posts and pages in the sidebar.', 'twentythirteen' ),
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h3 class="widget-title">',
		'after_title'   => '</h3>',
	) );
}
add_action( 'widgets_init', 'twentythirteen_widgets_init' );

and also in the templates there is <?php get_sidebar(); ?>.

I cant select widgets for my sidebar (which is also not rendered) under theme options –> customize (wp-admin/customize.php?theme=foo-bar) because the “Appearance” Option is missing. When I activate another theme, the option is present and I can select widgets for the sidebar.

Can anyone help me?

Best,
exchentrix