My custom metabox stops working on WP 4.9.8

Ok, I'm having some trouble updating my custom field to override the format_single_value method in the RWMB_Choice_Field. I'm extending the RWMB_Input_List_Field class because I want a list of checkboxes. Basically, I want to be able to pre-fill the checkbox options after a user as applied a Category to a CPT.

However, I'm getting Undefined index: multiple when I try to do that. I guess that makes sense because I'm using multiple values, but trying to override the format_single_value method. Is there another method in RWMB_Input_List_Field that I should be using instead?

Thanks in advance.

P.S. I can make a new post if that is preferred.

Thanks for troubleshooting this with me. I'm at the point now where I can see the fields I want in the admin... but when I try and save the post, I get a bunch of notices and the post won't save (Notice: Undefined index: clone in /phila.gov/wp/wp-content/plugins/meta-box/inc/meta-box.php on line 301) . Here's what I'm working with on the latest version of metabox:

if ( class_exists( 'RWMB_Field' ) ) {

  class RWMB_Unit_Field extends RWMB_Checkbox_List_Field {

    public static function normalize( $field ) {

      if( isset($_GET['post']) && get_post($_GET['post']) && isset($_GET['action']) && 'edit' == $_GET['action'] )

      $post = get_post($_GET['post']);

         if( isset($post)
             && !empty($post->post_type)
             && is_object_in_taxonomy($post->post_type, 'category')
         ){
            foreach ( (array) get_the_category($post->ID) as $cat ) {
              if ( empty($cat->slug ) )
                continue;

              //get data out of term meta field for use later
              $units[] = rwmb_meta( 'department_units', array( 'object_type' => 'term' ), $cat->term_id);
            }

          }

      if ( !isset( $units ) )
        return;

      foreach ($units as $key => $value){
        foreach($value as $name) {
          $options[] = array(
            'value' => urlencode($name['name']),
            'label' => $cat->name . ' - ' . $name['name']
          );
        }
      }

      $field = parent::normalize( $field );

      return $field;

    }

    /*Old, working version 
    public static function filter_options( $field, $options ) {
      $current_cats = get_the_category();
    
      foreach($current_cats as $cat){
        $units[$cat->name] = rwmb_meta( 'department_units', array( 'object_type' => 'term' ), $cat->term_id );
      }
    
      if ( !isset( $units ) )
        return;
    
      $options = [];
    
      foreach ($units as $key => $value){
        foreach($value as $name) {
          $options[] = (object) array( 'value' => urlencode($name['name']), 'label' => $key . ' - ' . $name['name']  );
        }
      }
    
        return $options;
     }
     */
  }
}

Thank you! I had to make a couple of modifications to what you posted, but your changes definitely helped! Posting the final(ish) version here in case someone else ever runs into this.

https://ghostbin.com/paste/ytadu