The measure is trying to tell you that your model already has an OutputDiagnostics object with the selection you are trying to add again, but the line is trying to access a variable new_diagnostic that hasn't been created in that branch of the code, so it's value is nil thus the getString method is undefined.

It's a bug in the measure, but you can just remove the measure from your workflow becasue the output it's trying to add has already been added.