Theme Customizer does not load when static Front Page is moved to Trash

[jeherve]

Steps to reproduce:

1. Go to Pages > Add New, and publish a new page
2. Go to Appearance > Themes > Customize
3. Under Static Front Page, set this new Page as your Front Page. Save changes
4. Go to Pages > All Pages, and trash the page
5. Go back to the customizer
   • The sidebar loads, but not the preview
   • I see the following error in the console: ​http://i.wpne.ws/MpDR

When commenting out $this->handle_404(); in wp-includes/class-wp.php#L550, the Preview appears again, but generates quite a lot of notices.

Tested with WordPress Trunk (Version 3.6-alpha-23400)

What happens on the front when you trash the front page? Seems like the customizer is loading fine, and it's probably showing the front just as it is.

Replying to helen:

What happens on the front when you trash the front page? Seems like the customizer is loading fine, and it's probably showing the front just as it is.

What happens in my testing is the request for all the customiser data gets the 404 page not found content not the customiser data content and the customiser never loads properly.

I guess I thought of it as the customizer loading fine, but I see that the front shows a 404 instead of just a white screen now (I was probably doing something weird earlier).

I also see a 404 page on the front.

• The customizer displays a white screen: ​http://i.wpne.ws/Mqlh
• The Reading Settings show that no front page was set: ​http://i.wpne.ws/MqFL

Confirmed using jeherve's steps. Live previews fail if you've trashed the static front page, but I also noticed that both the front page and the posts page have to be blank to trigger the problem.

Here's the error I see if "A static page" is selected and both the "Front page" and "Posts page" are blank in Settings → Reading:

Failed to load resource: the server responded with a status of 404 (Not Found)

Tested with WordPress trunk r21780.

If you leave "Front page" blank and select a "Posts page," the front page setting is ignored and the latest posts are displayed on the front end and in the customizer. Could the same behavior happen if both of those settings are blank?

Related: #24171