Look in the database for all activities that do not have an assigned object. Example (on dev):

Activity ID: 3452
Assigned comment ID: 1660

But it does not exist.

It should be one query that checks for such activities (or three queries for each activity type) and then 3 delete commands to remove such activities.

Later, when this is done, we need to check how come it was possible to delete a comment without deleting the activity and deal with such situations in the future.