Error on locked content by a deleted user

Problem/Motivation

I'm getting this error when I'm trying to edit a node that was locked by a deleted user: Call to a member function getDisplayName() on null in Drupal\content_lock\ContentLock\ContentLock->displayLockOwner() (line 186 of modules/contrib/content_lock/src/ContentLock/ContentLock.php).
Steps to reproduce the error:

1. Create a new user.
2. Edit a node with the new user.
3. With an admin user delete the previous user. (Try with the option delete the user and keep the content, otherwise you might get another error)
4. Try to edit the locked content that was being edited by the deleted user.

Proposed resolution

In order to prevent future issues a hook_user_delete() should release all locks from the user that is being deleted.
But for the existing locks, the table content_lock should be truncated or the entries should be deleted from those users.

delete from content_lock where uid = XXX;

or

truncate table content_lock;

Also you can use the devel module to run this, using the interface or a drush command

$uid = 2; // uid from the deleted user
$lock_service = \Drupal::service('content_lock');
$lock_service->releaseAllUserLocks($uid);