Ok, but still if you are debugging your application in debug mode (preferably using debug dcu's) the call stack at the moment of the error will indicate the exact line of the source code (together with call stack itself) where the error happened. You only mentioned the scripter error message, and the line in script that raised the error, but that script line is calling a Delphi method which in turn is calling Delphi code. It's inside that Delphi code that I'd like to see what's going on.
What is the relation of DefaultDrawColumnCell with __TDrawColumnEhCellEvent, for example?