I have been checking your file, but there is no indication on where the issue is happening? I guess it's in the script, but not sure. Can you please elaborate?
Note that IsNull() requires a parameter, either it be an expression or a field. If the field does not exist when you load it, then you get an error instead of a -1, not because the field is not null, but because it does not exist. Same may happen in the front end.
In addition to what Miguel points out, I just want to add that IsNull() when used in charts may in fact in some circumstances seem to return NULL: If you have missing values of type two, the expression is never calculated for these chart cells. So basically it looks as if IsNull() has returned NULL, when it in fact has not been calculated at all.
did you ever figure this one out? we have just upgraded from 10 to 11.2 and are facing a similar issue where if(isnull(FIELD_NAME) ,1,FIELD_NAME)) works in the client but on the access point it always returns null
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