Error: Warning: Invalid argument supplied for foreach() in element_children() (line 5043 of /var/www/drupal7/includes/common.inc

How to render a form in Drupal 7 with template.tpl.php?

<?php
// modulname.module

function modulname_permission() {
  return array(
    'administer_modulname' => array(
      'title' => t('Administer Modulname'),
      'description' => t('Administration for Text Size module.'),
    ),
    'access_modulname_content' => array(
      'title' => t('Access Modulname content'),
      'description' => t('Access of Text Size content.'),
    ),
  );
}
function modulname_block_info() {
  $blocks['modulname'] = array(
    'info' => t("Modul name 2")
  );
  return $blocks;
}
function modulname_block($op='list', $delta=0) {
  if ($op == "list") {
    $block[0]["info"] = t("Modul name 2");
    return $block;
  }
  elseif ($op == 'view') {
    if (user_access('access modulname content')) {
      $block['subject'] = t('Modul name 2');
      $block['content'] = drupal_get_form('modulname_form');
      return $block;
    }
  }
}
function modulname_block_view($delta) {
  $block['content'] = drupal_get_form('modulname_form');
  return $block;
}
function template_preprocess_modulname_form(&$variables) {
  $variables['modulname'] = array();
  $hidden = array();
  // Provide variables named after form keys so themers can print each element independently.
  foreach (element_children($variables['form']) as $key) {
    $type = $variables['form'][$key]['#type'];
    if ($type == 'hidden' || $type == 'token') {
      $hidden[] = drupal_render($variables['form'][$key]);
    }
    else {
      $variables['modulname'][$key] = drupal_render($variables['form'][$key]);
    }
  }
  $variables['modulname_form'] = implode($variables['modulname']);
}
function modulname_form() {
  $form = array();
  $form['modulname_select'] = array(
    '#type' => 'select',
    '#title' => "Modul name 2",
    '#name' => 'modulname_select',
    '#options' => array(
      '1' => t('One'),
      '2' => t('Two'),
    ),
    '#default_value' => variable_get('modulname_select', '1'),
    '#weight' => 0,
  );
  $form['modulname_submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
    '#weight' => 1,
  );
  $form['#skip_duplicate_check'] = TRUE;
  return $form;
}
function theme_modulname_form($form) {
  $output = drupal_render($form);
  return $output;
}

function modulname_theme() {
  return array(
    'modulname_form' => array(
      'variables' => array(
        'form' => NULL,
      ),
      'render element' => 'form',
      'template' => 'modulname-form',
    ),
  );
}

?>

// modulname-form.tpl.php
<?php print $modulname_form; ?>

// Error: Warning: Invalid argument supplied for foreach() in element_children() (line 5043 of /var/www/drupal7/includes/common.inc).

<?php
// Line 5043 in common.inc is: "foreach ($elements as $key => $value) {".

function element_children(&$elements, $sort = FALSE) {
  // Do not attempt to sort elements which have already been sorted.
  $sort = isset($elements['#sorted']) ? !$elements['#sorted'] : $sort;

  // Filter out properties from the element, leaving only children.
  $children = array();
  $sortable = FALSE;
  foreach ($elements as $key => $value) {
    if ($key[0] !== '#') {
      $children[$key] = $value;
      if (is_array($value) && isset($value['#weight'])) {
        $sortable = TRUE;
      }
    }
  }
  // Sort the children if necessary.
  if ($sort && $sortable) {
    uasort($children, 'element_sort');
    // Put the sorted children back into $elements in the correct order, to
    // preserve sorting if the same element is passed through
    // element_children() twice.
    foreach ($children as $key => $child) {
      unset($elements[$key]);
      $elements[$key] = $child;
    }
    $elements['#sorted'] = TRUE;
  }

  return array_keys($children);
}
?>