Warning: Invalid argument supplied for foreach() in element_children() (commerce_webform.module)

I've the following warning on Cart page having a product:

Warning: Invalid argument supplied for foreach() in element_children() (line 6433 of includes/common.inc).

I don't know exactly what cause the problem, but the following code generating this warning:

function commerce_webform_form_views_form_commerce_cart_form_default_alter(&$form, &$form_state, $form_id) {
...
  foreach (element_children($form['edit_quantity']) as $id) {

And NULL is passed into element_children().
Probably because my Quantity is read-only (just plain text, not textfield), so edit_quantity key doesn't exist?