The url to stealth forward to cannot be blank after a valid url has already been entered.
If the Stealth Forward name server option is not in use and you don't want the stealth forward url to be shown when you select your name server option, you can enter in another url to overwrite the previous url. This makes Stealth Forwarding your selected name server option, so right after, you can select the name server option that your domain should be using.
HOw to erase text entered in stealth forwarding