How to solve/debug get_terms suddenly showing no results?

Prior to 4.5.0, the first parameter of get_terms() was a taxonomy or list of taxonomies:
Since 4.5.0, taxonomies should be passed via the taxonomy argument in the $args array:

So, the lines

 /**
  * Get the family
  */
  $args = array(
      'parent' => $topmost->term_id,
  );
  $family      = get_terms($taxonomy_name, $args);  

should be

 /**
  * Get the family
  */
  $args = array(
      'taxonomy' => taxonomy_name,
      'parent' => $topmost->term_id,
  );
  $family      = get_terms($args);

See Documentation.

I hope this may help.