Take Text Editor for example.
reproduction steps.
1. Open Qt Creator. Tools->Options->TextEditor->Behavior.
2. check the "insert space instead of tabs".Then click "Apply" button. The "Apply" button itself didn't have any reaction. See Behavior.jpg.

Solution:
I think it is better to give the user some visual sense. For example we can disable the "Apply" button after it has been clicked. If the user make some changes we can enable the "Apply" button again.

Reason:
If the "Apply" button have some reactions, the user can know his operation has take effect. If not I convice that the user will click the "Apply" button not only once for he didn't know his operation is successful.