Reproduces in 8.0.0 as of commit 02969278aa (date: 9/17/2019)
First affected version is 6.9.1 release ("Set Active" didn't exist before then)
Repro steps:
1. Use a Universal project
2. Create a PBR shader and open it for editing
3. Create a new Unlit master node
4. Right click the new master node and click "set as active"
5. Press control Z until the new master node is gone
6. Save the graph
Expected result: undo operations should undo "set as active" actions, which would prevent this bug from happening
Actual result: since the active master node is now null, errors will be printed to the console on save. From this point on, that shadergraph, while still able to be opened, will be unusable; attempts to re-assign the master node or create new master nodes will fail and result in errors, and the shadergraph will not compile.