Add term of current custom post type to admin body class using admin_body_class

I am trying to add the current custom post type term to the body class of my WordPress admin page. So when I am viewing an existing custom post type that has been assigned a term it will add that term to the body class.

I have found the following code but cannot get it to work for me:

add_filter( 'admin_body_class', 'rw_admin_body_class' ); function rw_admin_body_class( $  classes ) {     if ( 'post' != $  screen->base )         return $  classes;      global $  post;     $  terms = wp_get_post_terms( $  post->ID, 'product_cat', array( 'fields' => 'all' ) );     $  terms = wp_list_pluck( $  terms, 'slug' );     foreach ( $  terms as $  term )     {         $  classes .= ' my_taxonomy-' . $  term;     }      return $  classes; } 

Any pointers on how to get this working?