Today, while installing a Wordpress Multisite application, I have started to see lots of this messages "Warning: Creating default object from empty value in wordpress". Those were appearing in the front-end of Wordpress, when activating a new site.
After searching a lot on Google for a solution, I have found none. Finally I have come to realize that this error is common, and I have seen before with another PHP application.
The whole problem was because I was using PHP 5.4, I switched to PHP 5.3 and the problem is now solved.