wp_insert_post() cannot set a page template from the frontend

[nacin]

wp_insert_post(), when setting a page template with 'page_template', has a dependency on the get_page_templates() function.

Since WP_Theme is now available in wp-includes, we can use wp_get_theme()->get_page_templates(). Note that the keys and values are flipped, so we need to change from an in_array() to an isset().

[nacin]

P2 was able to trigger this, it seems:

Call to undefined function get_page_templates()
File: wp-includes/post.php
Line: 2779
Source: wp-admin/admin-ajax.php?p2ajax=true

Not sure how this happened, though, because admin-ajax.php does include includes/admin.php, which includes includes/theme.php.

[nacin]

In [21847]:

Use wp_get_theme()->get_page_templates() rather than get_page_templates() in wp_insert_post(),
to allow page template setting on the frontend without a fatal error. fixes #21755.