$arg1 = array(
    'parent'                   => 0,
    'orderby'                  => 'name',
    'hide_empty'               => 0,
    'pad_counts'               => false
    );

$categories = get_terms('category', $arg1);  //or use your custom taxonomy name

if( !empty($categories) && is_array($categories) ){

  echo '<ul>';

  foreach($categories as $cat){

    echo '<li>'.$cat->name;

    $arg2 = array(
                    'child_of'                 => $cat->term_id,
                    'orderby'                  => 'name',
                    'hide_empty'               => 0,
                    'pad_counts'               => false
                );

    $sub_categories = get_terms('category', $arg2); //or use your custom taxonomy name

    if(!empty($sub_categories) && is_array($sub_categories)){

      echo '<ul>';

      foreach($sub_categories as $scat){

          echo '<li>'.$scat->name.'</li>';
      }
  echo '</ul>';
  }
  echo '</li>';
  }
echo '</ul>';
}