I've isolated the source of a fatal error that's been occurring when a brand new site is previewed just after installing and activating the plugin I'm working on.
The problem appears to be that the "category_children" item in the options table is not being created.
Interestingly, it appears that two actions trigger WP to create the category_children option. They are: (1) Activating a theme that contains code to create categories and (2) Simply accessing the category manager.
If I do either of those, after activating the plugin, the category_children option is being created (presumably due to checks in the theme activation or the edit-tags.php load) and the error goes away.
Here's the code I'm using in my plugin to create the categories (executes on plugin activation only):
if(!get_cat_ID('nofollow')){wp_create_category('nofollow',1);}
if(!get_cat_ID('noindex')){wp_create_category('noindex',1);}
What am I missing? Do I need to explicitly call a method to have WordPress generate the category_children options array for these items?
UPDATE: Thanks to scribu's help below on the reason for this bug, I've found that the PARTIAL fix for this bug is to place this line just after my wp_create_category() calls are complete:
clean_term_cache('','category');
However, this does not work when called in the same plugin that creates the categories. I had to place it in a standalone plugin that's activated separately from the plugin that creates my categories.