I'm using Vn3.0.5. of the plugin.
Error message = 'Notice: Trying to get property of non-object in ...\plugins\co-authors-plus\co-authors-plus.php on line 707'
1-Took existing user with a single post;
2-edited user to create profile for guest author;
3-edited post to assign to guest author (rather than user)
4-confirmed user had no posts, and guest author had one post..
5. (belt and braces), removed "WordPress User mapping" from Guest Author profile.
5-deleted user, ticked "delete any existing posts"
Prior to this, I had ticked "Assign any posts" to my user, and there was no error message. But then I found that I was being credited on the post as a co-author (which was not my goal).