math-type controls in 4.0 saved files may result in null template values
to see the problem you need a value with a default -- but which the user has never set. in 4.0 this would result in the default value storing in the custom map.
in 4.1 the custom map leaves the value out and backs off to default generation. however the backoff doesn't seem to be happening when the value is used as the argument to a math input.
the same thing works fine as an inline math template.