[4.0] Cannot save "Site Settings" in frontend

[PhilETaylor]

Steps to reproduce the issue

install 4.0-dev at 79fd949
install sample data
login as super admin on frontend
click site settings (to edit them)

Expected result

Save and close buttons work

Actual result

Save and close buttons do nothing.

System information (as much as possible)

79fd949
google chrome

[laoneo]

Please test #20620 which should fix the issue.