I just looked at that, and I like having that check in the validate method, but I'm not sure if it makes sense to modify the error message there.  validate() is a more general function that's used in a few places, so I don't want to change it's behavior too much.  I know in this particular use case we want to return a specific error message if the expression contains "instance(", but I'm not sure that we'd want to obscure the default message in all use cases.  

An alternative could be to alter the structure of `validate()`'s output to include result categorizations beyond a boolean, but that seems like it might be overkill.  What do you think?  If I move the check into validate, how else could I pass the result of that check to the function that updates the UI?

(good call on the variable for getExpressionFromUI(), I refactored a few things to do that, and I'll push changes once we decide on this issue)