Deleting a set front page should then display posts on front page

[hailin]

Create a new page.

Go to Options > reading and set that page as the front page.

Now delete that new page

Front of the blog goes blank.

Expected behaviour: It defaults back to Posts.

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Hailin:
The root cause is the following: Suppose option show_on_front is configured to "page", and page_on_front is set to a specific page_id, and/or page_for_posts is set to another page_id.

Then the user deletes a page from Manage=>Page, which happens to be the configured page_on_front. Then the page is gone, but those options still hold that deleted page_id. That cause the reported problem.

The fix is that when we delete a page, we check to see whether the page is used in any of those options. If yes, we adjust them as well.

[hailin]

patch file

[ryan]

(In [6337]) Update page on front settings when deleting the page on front. Props hailin. fixes #5361