As a general observation when changing settings in Settings page (AdminPagePresenter), is that invalid inputs are handled independently from the save operation.
Namely, the user can enter whatever value in input filed they want and click Save button, which drives them away from the page with the notification message saying that the change was successfully saved (although the error message pops up below the input field in case of an invalid input for just a moment before the page is closed and the value is left unchanged).

I would suggest moving the input validation on save and prevent leaving the page until a valid value is entered. In this way, the erroneous input can be rectified immediately during the same visit of the page and not only after realizing the input entered hasn't been propagated as a new setting.