Error while adding an Extension field

Hi All,

While adding an extension attribute to extension definition, I am getting below error.

This object has been changed by another user, and your changes cannot be saved.\nYou must cancel these changes and reload the document.

Actually, I addded one attribute and when I was adding the other attribute, I saved the extension definition without specifying the
Value list. And I got this error. And now even when I give the value list name, its throwing above error.

Please guide me how to resolve this error.

Thanks,

Saloni