[wp-meta] [Making WordPress.org] #1731: Fix notice for function wporg_is_handbook on single post pages

[Making WordPress.org]

#1731: Fix notice for function wporg_is_handbook on single post pages
--------------------------+-----------------------
 Reporter:  keesiemeijer  |      Owner:
     Type:  defect        |     Status:  new
 Priority:  normal        |  Milestone:
Component:  Handbooks     |   Keywords:  has-patch
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 The function `wporg_is_handbook()` displays this notice on single post
 pages when used before the queried object is set (as in the pre_get_posts
 action).

  Notice: Trying to get property of non-object in /srv/www/wp-
 reference/public/wp-includes/query.php on line 4654

 This notice originates from `WP_Query::is_singular()` where
 `get_queried_object()` is used.

 To fix this warning there needs to be a check if the queried object is
 set. If it's not set, use the post type query var from the current query
 to determine if it's a handbook page.

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Ticket URL: <https://meta.trac.wordpress.org/ticket/1731>
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