Deleting all users without id

Problem/Motivation

I tried to delete over 50,000 users at once by executing drush duda --yes. The batch started but always got stuck after some random amount of users.

Steps to reproduce

1. run drush duda --yes to delete a lot of users (at least 50 I guess)
2. see the error "Call to a member function delete() on null ..." (line 61 of delete_all_users_batch_delete in delete_all.users.batch.inc)

Proposed resolution

Verify the user account exists before deleting it.