I think that the problem is that, without explicitly adding users to a note, the “target users” list that is sent to the query is null (empty list).
In the install portion of plugin.php, this line is included in the db table create code:
target_users varchar(200) NOT NULL
So, the db table requires non-null values for target_users, but the code uses an empty list to indicate that there are no users explicitly associated with a note.
Anyone know how to fix this? I’m a php WordPress newbie.