closes: https://github.com/TryGhost/Ghost/issues/15505
When starting Ghost with a missing active theme, the design settings screen and change theme screen both end up in a broken state with the user unable to select a new theme as the active one.
The design screen has no default (or blank) slate, and so shows a preview of an empty theme.
- First added a new default screen to serve as a placeholder for when the state contains no active theme.
- Added a check for when there was no active theme, then redirects the user to the default screen .
The change theme screen wants to set an active property on the theme that should be active in the theme list.
- Added a check to see whether there is an active theme set.
- If there isn't one, don't bother trying to add the active property.
