map_meta_cap PHP Notices when Post type is no longer registered

[dd32]

Noticed in the recent comments widget:

( ! ) Notice: Trying to get property of non-object in C:\www\wordpress-commit\wp-includes\capabilities.php on line 929
Call Stack
#	Time	Memory	Function	Location
1	0.0005	360328	{main}( )	..\index.php:0
2	0.3960	24624984	wp_dashboard( )	..\index.php:63
3	0.3960	24625272	do_meta_boxes( string(9), string(6), string(0) )	..\dashboard.php:221
4	0.8141	25014760	call_user_func ( string(28), string(0), array(4) )	..\template.php:970
5	0.8141	25014776	wp_dashboard_recent_comments( string(0), array(4) )	..\template.php:0
6	0.8171	25271192	current_user_can( string(9), string(3) )	..\dashboard.php:632
7	0.8171	25271784	call_user_func_array ( array(2), array(2) )	..\capabilities.php:1047
8	0.8171	25272048	WP_User->has_cap( string(9), string(3) )	..\capabilities.php:0
9	0.8172	25272448	call_user_func_array ( string(12), array(3) )	..\capabilities.php:729
10	0.8172	25272736	map_meta_cap( string(9), string(1), string(3) )	..\capabilities.php:0

Turns out, that it's a comment on a custom post type that's no longer registered.

As a result $post_type = get_post_type_object( $post->post_type ); fails to return a valid post type object (however, get_post() can still return the cpt) which results in the code operating on a null object.

[dd32]

#16956