Steps to reproduce:

1. Create fre-FR language.
2. Use the following Site Access config in .yamls:

           site:
               languages: [eng-GB]
           fre:
               languages: [fre-FR]
   

       system:
           admin_group:
               languages: [eng-GB]
               page_builder:
                   siteaccess_list: [site, fre]
   

3. Turn off alwaysAvailable flag in Landing Page Content Type.
4. Create Landing Page in eng-GB.
5. Translate this content into fre-FR.

Result:
One is presented with an empty screen. Preview action throws 500.

Expected result:
A preview should be rendered properly.