If the Platform UI is accessed via Host (using Map\Host siteaccess matcher), then Landing Page preview doesn't work correctly.

After clicking "Page" tab in the Admin UI, the preview iframe is loaded, but inside the iframe there is another instance of Admin UI loaded instead of correct Landing Page preview.
From the menu on top, it is still possible to click on another siteaccess (different than the default one) or on "eZ Studio presentation" button and then the preview will work correctly.

If the Editor goes to "Content"/"Content structure" first, selects a Landing Page, and then clicks on "Page", the following error is displayed (in dev mode):

Twig_Error_Runtime:
Variable "siteaccess" does not exist.

  at vendor\ezsystems\studio-ui-bundle\Resources\views\page\page.html.twig:63

The issue might also happen when using other siteaccess matchers.

Steps to reproduce

1. Create a new eZ Platform v2 installation.
2. Add the following configuration to your ezplatform.yml file:

   ezpublish:
       siteaccess:
           match:
               URIElement: 1
               Map\Host:
                   admin.your-site.example: admin
   

3. Access your site via admin.your-site.example. Admin UI should be shown. Log in.
4. Click on "Page". The loaded iframe will have another Admin UI instance instead of Landing Page preview.
5. Go to "Content"/"Content structure" and then click on "Page" again. The error described above will be shown (in dev mode). In prod mode, there will be the same symptoms as in step 4