Hey jwfitt -
Try the same test, but this time double clicking the Default State Machine, selecting all the nodes in that Machine except the entry node and then copy them. Go back to your AnimGraph and double click on the new state machine, and paste the copied nodes here. Assuming it does not crash, attach the Entry node to the first Idle node.
There is a known issue with trying to nest State Machines within one another.
Let me know if this is a possible solution for you,
-Eric Ketchum