Auto set Featured Image from Relationship Media Field

I’m trying to automatically set Featured Image upon saving/updating custom post type. Image should come from Relationship Media Field where user can upload pictures. It should set 1st image uploaded as a feature image.

Below code doesn’t work and I’m not sure why.
Custom post type name: car_listing
Relationship Media field: images

add_action( 'pods_api_post_save_pod_item_car_listing', 'slug_auto_featured_img', 10, 3 );
function slug_auto_featured_img( $pieces, $is_new_item, $id ) {
	//get the values of the 'images' field
	$imgs = $pieces[ 'fields' ][ 'images' ][ 'value' ];
	//get ID of first index in array
	$img = $imgs[0][ 'ID' ];
	//if there is nothing there set $img to null to avoid errors
	if ( empty( $img ) ) { $img = null; }
	//Prepare to update the post's featured image.
	$my_post = array(
		'ID'              => $id,
		'post_thumbnail'  => $img,
	);
	//Update the post
	wp_update_post( $my_post );
}