"FIXME: Close the OSPipes now, otherwise the image will block on a read"
[self complete] fork "defer execution so OSPipes stay in place for now"]
Apparently the call to self#initialStdErr fails to return an object and nextPutAll is sent to a Nil object, which doesn't understand that selector.
In general the code in that method looks dodgy (several "FIXME" comments) but I'm not clear if its Aida code or Pharo code... looks like a Pharo problem but I gather it is a good idea to report it here since Aida is affected by it on Pharo for Windows.
In 6.5 interim a hourly snapshot is disabled by default, so you don't
have issues with OSProcess on Windows anymore. In the meantime I hope
Pharo guys will solve the problems with OSProcess on Windoze.