SIGSEGV, possibly on niGui

[KnorrFG]

Hey, this is somewhat of a cross post. The original can be found here

To sum it up: I wrote a small program, that renders graphs based on some text that is parsed. The code can be found in this repo. If I try to apply the first parser test by hand in the gui version (adding nodes 3, 4 and 5 successively to the session "Lala") it will produce a SIGSEGV. If you play around for a minute or two with the program it will crash at different points. The stack traces are in the first post of the original thread.
I also wrote a small test for niGui, which does not crash. So the gui and the parser seem to work fine by themselves and only crash when put together. This happens on Windows and on Linux. However, when I add --gc:arc the crashes are gone. On both platforms. I was told in the original thread, that this is probably a bug in niGui and was advised to post it here.