Cannot delete previously created widget set

The "delete" option for an user-created widget set doesn't work. When you try to delete a widget set, Drupal returns to the widget set list displaying an "undefined index" notice, and the set will remain on the list (even though it says "Set X was deleted").

The cause of this problem is a missing "&" in the widgets_set_delete_form() function prototype. The parameter $form_state is passed as a value, so the form doesn't register the widget set reference and nothing is deleted.

Solution:

To fix this, simply edit line 492 of widgets.admin.inc:

function widgets_set_delete_form($form, $form_state, $set) {

...and add the missing & to $form_state:

function widgets_set_delete_form($form, &$form_state, $set) {

Hope this helps anybody.