Steps to reproduce:

1. Create a new project
2. Import the attached "MyEnumScript" script to the project
3. Install the Behavior package from Package Manager > Unity Registry
4. Create and open a new Behavior Graph
5. Create a new Action node that takes the enum as parameter (in Enumeration sub-menu or search by name)

Actual results: 

• Exceptions spam: `InvalidCastException: Specified cast is not valid.`
• The graph view is not rendered and is unusable

Expected results: 

• No exception raised and graph view usable

Reproducible with versions: 1.0.9 (6000.0.40f1)
Not reproducible with versions: 1.0.8 (6000.0.40f1)

Tested on (OS): Win, MacOS (M1)