Hey airspan,
The first thing I would look to do is put a watch on that expression, Logging.StepResult.Numeric, to make sure it exists and can be accessed in the scope that is calling it. The option is in Debug > Breakpoints/Watches, then add the watch expression.
It might also be helpful if you could post your code or message it to me so I could see what is going on myself. You can also email support@ni.com to get further help on the issue.
Sam J.
Applications Engineer
National Instruments