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. 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 Page Builder.

Result:
You will be presented with an error:

Could not find 'Content' with identifier 'array (
'id' => 64,
'languages' => NULL,
'versionNo' => NULL,
)'

Exact same issue was fixed in Landing Page Editor: EZEE-1795.