Behavior component throws null reference errors and the graph is broken when referencing a blackboard variable from a deleted script

Steps to reproduce:

1. Create and open an empty project
2. Install com.unity.behavior
3. Create and open a behavior graph (Assets > Create > Behavior > Behavior Graph)
4. In the blackboard, create an enum variable
5. Delete the enum script that got created
6. Create a game object with a behavior graph agent and assign the behavior graph to the agent

Actual results: Null reference errors are thrown and the behavior agent component is not displaying the graph variables

Expected results: No errors are thrown and the behavior agent component displays the graph variables correctly

Reproducible with versions: 
1.0.7
1.0.1

Tested on (OS): MacOS

Notes:

• Happens with other things than enums as well
• Regenerating the graph fixes it