A PHP notice is generated in :
1. log in as admin in admin interface
2. Click "Content structure"
3. Click "Create new" button
4. Select a class which has the Author datatype in the drop down
5. When editing the object, current logged in user will be proposed as Author
6. Click the check box next to the proposed user and click "Remove selected"
7. Click "Send for Publishing"
8. Validation of the object will fail because of missing author, you are back at the editing page
9. Click "Add author"
10. The page will reload with the mentioned PHP notice