Re: Warning: Attempt to modify property of non-object


5.0 series is mainly focused on Font Awesome new series. So,

If you have used icon codes in your blog than you ***must*** change them to the new icon codes.
We always recommend to customize your theme without changing the style.css. It can be done via Extend Settings. If you have not made any customization to the core files of our themes than you have no worry about upgrade.