Clone group inside MB Block

  $meta_boxes[] = [
    'title'           => 'Button',
    'id'              => 'btn',
    'description'     => 'Inserts buttons that are either stacked or inline, primary or standard styling.',
    'type'            => 'block',
    'icon'            => 'admin-links',
    'category'        => 'formatting',
    'keywords'        => ['button', 'link', 'action', 'cta'],
    'render_template' => get_template_directory() . '/blocks/btn.php',
    'enqueue_style'   => $wp_admin_css,
    'supports'        => [
      'align' => ['wide', 'full'],
    ],
    'fields'     => [
      [
        'id'         => 'btn_group',
        'type'       => 'group',
        'clone'      => true,
        'add_button' => '+ Add another button',
        'fields'     => [
          [
            'id'   => 'btn_txt',
            'type' => 'text',
            'name' => 'Button text'
          ],
          [
            'id'   => 'btn_url',
            'type' => 'text',
            'name' => 'Button link'
          ],
          [
            'id'      => 'btn_stacked',
            'name'    => 'Stacked',
            'type'    => 'select',
            'options' => array(
              '1' => 'Yes',
              '0' => 'No'
            )
          ],
          [
            'id'      => 'btn_style',
            'name'    => 'Style',
            'type'    => 'select',
            'options' => array(
              'primary'  => 'Primary (pink)',
              'standard' => 'Standard (blue)'
            )
          ]
        ]
      ]
    ]
  ];