When selecting an non-closed figure, draw e.g. with the measurement "Draw paths" tool, a red message appears inside the statistics view (same label as normally used for calculating message) notifying: "Exception occurred. See log for details".
Inside the Log window you can find:
Error Runtime Exception: Masking not possible for non-closed figures
May be good to catch exceptions but not very useful for end-time users...
Probably there should be an QMessageBox or something equal!