Possible to add more items to a scheduled project

Steps to reproduce

Steps to reproduce

1. Existing 2 admin users
2. On Project gaget > create project 1 > Add Page 1 in "ready to publish" to project 1 
3. in browser 1: Admin 1 is is opening the 'Schedule Project for Publish' of project 1
4. in Browser 2: Admin 2 is is opening the 'Schedule Project for Publish' of project 1 > select specific time > set schedule publish for project 1
5. in browser 1: Admin 1 selects time > click Schedule >>> an message show "The project is no longer in an editable state. It may have been changed by someone else." ( Right) > Click OK button
6. Admin 1 dnd some contents (page,.) to Project 1