This error message only occurs when you double click a project file, not with File - Open in the program. After that, the file is opened correctly.
This message occurs already before the first line of our code is reached. It occurs as well with other programs, like e.g. AutoDesk DWG TrueView sometimes with MS Word. The reason seems to be outside of our software.
As the file is opened correctly after the message, this error message can be ignored.