Integrity constraint violation error while allowing users to register using username in drupal commons

Hi,

I'm using drupal commons and I want to allow user to pick his username.

Currently drupal commons does not allow user to choose his own username but generates one automatically.

What I have done so far through hook_user_register_form_alter :

• I changed the #type of name (username machine name) from hidden to textfield. This display the field.
  

  $form['account']['name']['#type'] = 'textfield';
    

• I removed the default value
  

  $form['account']['name']['#value'] = NULL;
   

Now when I try to add a new user, I get the following error message:

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'name': INSERT INTO {users} (uid, name, pass, mail, signature, signature_format, created, status, timezone, language, picture, init, data, uuid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13); Array ( [:db_insert_placeholder_0] => 824 [:db_insert_placeholder_1] => [:db_insert_placeholder_2] => $S$DgHF.OcL6K0hxQ0A4v.O67r4FUp4kpfeuywOrGHvb4zmoEzjpQrW [:db_insert_placeholder_3] => test@test.com [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => filtered_html [:db_insert_placeholder_6] => 1463562222 [:db_insert_placeholder_7] => 1 [:db_insert_placeholder_8] => Europe/Brussels [:db_insert_placeholder_9] => en [:db_insert_placeholder_10] => 0 [:db_insert_placeholder_11] => test@test.com [:db_insert_placeholder_12] => a:2:{s:7:"contact";i:0;s:20:"l10n_client_disabled";b:0;} [:db_insert_placeholder_13] => 267c9864-8fe0-4cc5-81ef-22bd53befcf8 ) in drupal_write_record() (line 7333 of /home/me/websites/customprojects/mysite.local/docroot/includes/common.inc). Backtrace:
PDOStatement->execute(Array) database.inc:2171
DatabaseStatementBase->execute(Array, Array) database.inc:683
DatabaseConnection->query('INSERT INTO {users} (uid, name, pass, mail, signature, signature_format, created, status, timezone, language, picture, init, data, uuid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13)', Array, Array) query.inc:36
InsertQuery_mysql->execute() common.inc:7333
drupal_write_record('users', Object) user.module:582
user_save(Object, Array) user.module:3912
user_register_submit(Array, Array) form.inc:1519
form_execute_handlers('submit', Array, Array) form.inc:903
drupal_process_form('user_register_form', Array, Array) form.inc:385
drupal_build_form('user_register_form', Array) form.inc:130
drupal_get_form('user_register_form') 
call_user_func_array('drupal_get_form', Array) menu.inc:527
menu_execute_active_handler() index.php:21

I understand that it is a duplicate problem on the username field but checking in the database there is no username matches the one I have chosen.
So don't understand the error.