Hi,
good evening,
Seems like a bug - your gui sending "no null" , when you trying to delete the value of the user idle timeout....and the right command should be "no user-idle timeout"
What you should do:
*login via ssh/console
*enable password
*conf t
*aaa authentication captive-portal *your captive profile*
*no user-idle timeout
*write mem
Enjoy :)
BTW: Report here what AOS you are using - in order for this issue to be fixed.