[GREENFOOT-540] Multiple Greenfoot instances can overwrite properties on exit
On Windows and Linux, you can open multiple instances of Greenfoot (e.g. by double-clicking two project files in file explorer on Windows). In this case, they will overwrite each other's properties files on exit, which is primarily problematic on first load, because it can overwrite the UID/GUID [for data recording] which have been generated on loading.
A minimal solution is to write the properties immediately when we generate a UID or GUID.
A more comprehensive solution may be to add singleton functionality that doesn't permit more than one instance at a time (a bit like happens on Mac).
- Issue type: Bug
- Priority: Medium