Unfortunately, an ugly bug crept into Reason 12.5.2.
The bug makes Reason 12.5.2 crash if you try to automate any Master Section parameter - or create a sequencer track for the Master Section device.
We're working to fix this bug a.s.a.p. and plan to release a bug fix update late next week.
We're terribly sorry for the inconvenience!
Meanwhile, don't automate any Master Section parameter - or create any sequencer track for the Master Section.