Assigning graphs on prefabs does not work

Steps to reproduce:

1. Create and open an empty project and install com.unity.behavior
2. Create a behavior graph (Assets > Create > Behavior > Behavior Graph)
3. Create a game object with a behavior agent
4. Assign the graph to the behavior agent
5. Make a prefab out of the game object by dragging it to the project view
6. In the project view, i.e. prefab asset, set the graph to none
7. Click on the prefab game object in the hierarchy and observe the graph assigned to the behavior agent

Actual results: the graph field is still assigned to the graph asset

Expected results: the graph field is none

Reproducible with versions: 1.0.8

Tested on (OS): MacOS

Notes:

• A workaround seems to be to make edits in prefab isolation mode
• This is also an issue for the blackboard variables
• Reported by a user
• There is a slight difference in behavior between versions, but this never properly worked.
• Assigning in the hierarchy is also a problem that needs to be fixed, where it goes back to the graph used in the prefab (another user reported this).