Error at 'Add Media' page: Undefined index: media_add_upload in drupal_retrieve_form

function workbench_media_add($form, &$form_state) {
  $form = drupal_get_form('media_add_upload', $form, $form_state);
  $form['#submit'][] = 'workbench_media_add_submit';
  return $form;
}

The "media_add_upload" form not longer exist in my Media 7.x-2.* . It use to exist in Media 7.x-1.2 in media/includes/media.pages.inc.

What version of the Media module are you using, and do you have a meda_add_upload function?

I had a bad mix of media and workbench_media versions, a 7.x-2 and a 7.x-1.0, respectively.

The current branch of Media you are using should be the same branch of Workbench Media that you should use. For example, if you are using the Media 7.x-1.0 release, then you should use the latest Workbench Media 7.x-1.X release.

Need an explicit dependency of the version of Media module in the workbench_media.info file.
In Drupal 7 "workbench_media.info" file, dependencies[] = media (>=2.x)

This will be fixed by #1922706: Change dependency from Media to File entity.