[Release] Mac release broken? ClassBinding>>objectForDataStream

For both 4.6 & 5.0 on the mac, saving a project generates the following error: "NotImplemented: ClassBinding or a superclass should implement objectForDataStream."

The comments in the method suggest this shouldn't have been released to a stable version: "It's not yet clear how serialization should work in the presence of environments".

Problem 2 is that if I then "abandon" the error messge, the workspace is then flooded with "MessageNotUnderstood" error boxes.

Is "environments" something experimental that should be switched off?

Best, Huw