Fix crash when "Wait for validation" is not enabled

Qt is not happy that we're deleting a radio button from a slot connected
to QAbstractButton::toggled, which makes sense, since it's probably not
finished internally, so now we do a delayed invokation of
boxAsker::checkAnswer that is the method that may ultimately end up
deleting the radio button, but this way it runs when Qt is finished
processing the toggled change

BUGS: 435555