Avoid panic by only restoring workspace if UI has launched (#18386)

This should fix the `unregistered setting type
workspace::workspace_settings::WorkspaceSettings` panic that came from
inside `restorable_workspace_locations`.

We tracked it down to a possible scenario (we can't recreate it though)
in which `app.on_reopen` is called before the app has finished
launching.

In any case, this check makes sense, because we only want to restore a
workspace in case the whole app has launched with a UI.

Release Notes:

- N/A

Co-authored-by: Bennet <bennet@zed.dev>