Steps:

1. Log in to the back office
2. Go to Admin/Roles
3. Click Editor
4. Switch into Assignments tab
5. Click Assign to Users/Groups button
6. Select Editor in User Groups
7. In Limitations section select Sections, choose Standard for example.
8. Click Save and close button

Actual result:

Warning: "Define a Subtree or Section limitation" is displayed. If user clicks Save and close button again, it will work.

Expected result:

Limitations are saved, form is closed without any warning.