In some cases, regenerating a graph does not make the blackboard lose its reference to it

Steps to reproduce:

1. Create a behavior graph "SubGraph" (open it and close it so that it will generate the runtime graph)
2. Create and open a blackboard asset
3. Add a BehaviorGraph variable to the blackboard asset
4. Create and open another behavior graph "Parent"
5. Dock the blackboard asset window next to the behavior graph window
6. Change to the behavior graph tab
7. Add the blackboard asset to the "Parent" behavior graph
8. In the project view, select "SubGraph"
9. In the inspector, click on the "Regenerate Runtime Assets"
10. Open the blackboard asset and observe the value of the BehaviorGraph variable

Actual results: The BehaviorGraph variable still references the subgraph

Expected results: The BehaviorGraph variable shows a missing reference

Reproducible with versions:
1.0.9

Can’t test with versions: 
1.0.8 (the regenerate button is not there)

Tested on (OS): MacOS

Notes:

• Notice that the behavior graph window shows a missing reference
• If you leave the blackboard asset window shown open, the blackboard will actually reference a missing variable