Steps to reproduce:

1. Log in to back office.
2. Go to Admin / Sections (v3.3)  <>  Content structure / Sections (v4.0 and v4.1)
3. Start creating a new section.
4. In field Identifier provide special characters e.g @@@@@
5. Publish.

Actual result:

See attached screenshot.

Missing validation popup message for creating new Sections.

Expected result:

Validation popup message is displayed.

Note:

On v4.0 and v4.1 the validation message is displayed but twice.