Use of an unexisting array index

I rewritten the function containing that code as follow:

function xmlsitemap_node_form_alter(&$form, &$form_state, $form_id) {
  if (isset($form['type']) && $form_id == $form['type']['#value'] .'_node_form') {
    $node = $form['#node'];
    if (user_access('override node priority')) {
      $form['xmlsitemap_node'] = array(
        '#type' => 'fieldset',
        '#title' => t('Site map settings'),
        '#collapsible' => TRUE,
        '#collapsed' => TRUE,
        '#weight' => 30,
      );
      $options = xmlsitemap_priority_options('both');
      $default = variable_get("xmlsitemap_node_type_priority_$node->type", '0.5');
      $form['xmlsitemap_node']['priority_override'] = array(
        '#type' => 'select',
        '#title' => t('Site map priority'),
        '#default_value' => $node->priority_override,
        '#options' => $options,
        '#description' => t('The default priority is %priority.', array('%priority' => $options[$default])),
      );
    }
    else {
      $form['priority_override'] = array('#type' => 'value', '#value' => $node->priority_override);
    }
    $form['xmlsitemap_node_status'] = array('#type' => 'value', '#value' => $node->status);
  }
  else {
    switch ($form_id) {
      case 'node_type_form':
        if (isset($form['identity']['type'])) {
          $form['xmlsitemap_node'] = array(
            '#type' => 'fieldset',
            '#title' => t('Site map'),
            '#collapsible' => TRUE,
            '#collapsed' => TRUE,
          );
          $form['xmlsitemap_node']['xmlsitemap_node_type_priority'] = array(
            '#type' => 'select',
            '#title' => t('Priority adjustment'),
            '#default_value' => variable_get("xmlsitemap_node_type_priority_{$form['#node_type']->type}", 0.5),
            '#options' => xmlsitemap_priority_options('exclude'),
            '#description' => t('This number will be added to the priority of this content type.'),
          );
          $form['xmlsitemap_old_priority'] = array('#type' => 'value', '#value' => variable_get("xmlsitemap_node_type_priority_{$form['#node_type']->type}", 0.5));
          $form['#submit'][] = '_xmlsitemap_node_submit';
        }
        break;
      
      case 'xmlsitemap_settings_sitemap':
        $form['xmlsitemap_node'] = array(
          '#type' => 'fieldset',
          '#title' => t('Content priority'),
          '#description' => t('The default priority for specific content types can be set on the !link pages.', array('!link' => l(t('content type settings'), 'admin/content/types')))
        );
        $form['xmlsitemap_node']['xmlsitemap_node_promote_priority'] = array(
          '#type' => 'select',
          '#title' => t('Promotion adjustment'),
          '#default_value' => variable_get('xmlsitemap_node_promote_priority', 0.3),
          '#options' => xmlsitemap_priority_options(),
          '#description' => t('This number will be added to the priority of each post that is promoted to the front page.'),
        );
        $form['xmlsitemap_node']['xmlsitemap_node_comment_priority'] = array(
          '#type' => 'select',
          '#title' => t('Comment ratio weight'),
          '#default_value' => variable_get('xmlsitemap_node_comment_priority', 0.5),
          '#options' => xmlsitemap_priority_options(),
          '#description' => t('This number will be multiplied by the ratio of the number of comments on a post to the highest number of comments on any post—that is, this number will be added to the priority of the post with the most comments.'),
        );
        $form['xmlsitemap_node']['xmlsitemap_node_count_comments'] = array(
          '#type' => 'checkbox',
          '#title' => t('Count comments in change date and frequency'),
          '#default_value' => variable_get('xmlsitemap_node_count_comments', TRUE),
          '#description' => t('If enabled, the frequency of comments on a post will affect its change frequency and last modification date.'),
        );
        $form['buttons']['#weight'] = 1;
        break;
    }
  }
}

Fixed in 6.x-1.x-dev.