After removing the user that created a Landing Page Content Object, this Landing Page is no longer editable.

Steps to reproduce

1. Create a new installation of eZ Platform EE.
2. In the Platform UI, create new User in the "Editors" User Group with login "test_editor".
3. Log out and log in again as "test_editor".
4. Under "Home", create new Landing Page Content Object named "Test Landing Page".
5. Log out and log in again as "admin".
6. Delete User "test_editor".
7. Open "Test Landing Page" in Landing Page Editor.
8. You will be presented with an error "Cannot load user data". The Landing Page Editor will not be responsive.

NOTE: not having permissions to read the creator's data also results in the same error. The creator doesn't have to be deleted.