The MBS plugin have several really good functions and one the once that I like the most is one that FileMaker is totally missing out on and that should have been implemented long time ago.
If you are trying to use a Variable and it has not been declared earlier in the script the Script Row in Script Workspace should give us a Red flag or something equal to that. Find this kind of errors is really hard and if I did not have MBS plugin today I would most probably have more errors in my code then what I have today.
What I suggest is that we get some kind of warning. Red colors is always a warning.
This function would only work for local variables since global variables could be declared in another script or function.