I only tried this on macOS.

Steps to reproduce:
1) Open Qt Creator Settings. This should open you directly to the Kits settings pane.
2) Try to click Add button on the right to add new kit. Nothing happens, except the left Kits list widget seems to lose focus (goes from blue to gray).
3) Click Add button one more time, it works.
4) Click Add again, it doesn't work.
5) Click Add again, it works.

Same behavior with other buttons like Cancel and Ok for the general settings.

Can reproduce with 4.13.0-beta1 and beta2 (both are using Qt 5.15.0).
Everything works fine with official 4.12.3 installed from Qt installer (Qt 5.14.x)

Most likely a 5.14.x -> 5.15.0 regression.