Fatal error on CB Activity

I've no idea, that's something the developers of DT need to review further. If they're going to delete a user then it must be done with that function so as long as you replaced their delete usage with that function everything should work fine from then on.

I have to upgrade this.

Adding the last part of code didn't work. The only way it work is adding this:

$_PLUGINS->registerFunction( 'onAfterDeleteUser', 'userDeleted', 'getDemoTab' );

Anyway, in the activity list in backend I still see activities of user deleted but no owner is associated.

You're just acting on the delete user trigger that CB fires. That has nothing to do with actually deleting the user. Please review my previous posts as I've explained what function you need to use for DT to properly delete a user.

CB Activity doesn't currently delete a users activity when the user is deleted, but it won't show on frontend anymore. Have added a feature ticket to implement this for next release. It initially was not implemented to avoid clearing a database of a users activity if they were accidentally deleted for whatever reason.

forge.joomlapolis.com/issues/3874