title: django-allauth + django-invitations: IntegrityError at /accounts/signup/, duplicate username
body: <p>I am using django-allauth + django-invitations. When I access the url in the emailed invitation sent from django-invitations, I receive this error.</p>
  <pre><code>IntegrityError at /accounts/signup/
  duplicate key value violates unique constraint &quot;accounts_userplus_username_key&quot;
  DETAIL:  Key (username)=() already exists.
  Request Method: POST
  Request URL:    http://127.0.0.1:8000/accounts/signup/
  Django Version: 4.0.5
  Exception Type: IntegrityError
  Exception Value:    
  duplicate key value violates unique constraint &quot;accounts_userplus_username_key&quot;
  DETAIL:  Key (username)=() already exists.
  </code></pre>
  <p>UserPlus is my custom model name</p>