parallelstream must not do an addAll to an ArrayList: it causes null entries

[udatny]

https://github.com/miho/JCSG/blob/5801d917460012438412d0cd8e5fa2c2697c1bd4/src/main/java/eu/mihosoft/jcsg/Edge.java#L609

https://stackoverflow.com/questions/39093178/java-8-parallelstream-fill-arraylist

[miho]

Thanks for spotting this. There are probably a couple of other problematic locations.