My code :
var descr = new string('@Unique(@DbLookup("":"";"":"test/demo.nsf";"searchvg";"ARKD')+types[i]+('N";4))');
description = @If(@IsError(sessionAsSigner.evaluate(descr)), "no data", sessionAsSigner.evaluate(descr));
When there's no error , the variable description will get the value of the lookup. When the data of the lookup doesn't return a value , the variable description doesn't get the value "no data", and I get an error message :
Exception occurred calling method NotesSession.evaluate(string) null