Which version of WordPress Mu are you running? Which version of BuddyPress?
Hi,
I have got the same problem. I tried upgrading Buddypress to 1.0.3 and got the same fatal error.
I’ve tried replacing all the files in the plugin folder but that didn’t work.
Someone help please!!!!
You need to be using WPMU 2.8.* for BP as the way widgets are written was changed.
Getting out of a fix:
If you’ve tried updating Buddypress, but forgotten to update WordPressMU first of all… you are now facing the “Fatal error: Class ‘WP_Widget’ not found”.
You’ll also find, you can’t rewind your buddypress upgrade, and can’t update wordpress from the dashboard. You are stuck!
You could reach for your backup, but if like me, you got lazy and didn’t backup… you’ll be really pleased to know this link to a manual update process works, and you won’t lose any of your lovely data:
https://codex.wordpress.org/Upgrading_WPMU#2.7.1_to_2.8.3
The steps for 2.8.3 also work for 2.8.4a
