Reproduction steps:
1. Create a new 3D project
2. Install the "Behavior" package via the "Package Manager" ("Window -> Package Manager")
3. Create a new "Behavior Graph" ("Assets -> Create -> Behavior -> Behavior Graph")
4. Open the created behavior graph
5. In the "Blackboard", press the '+' symbol in the top-right and select "GameObject", name it "Test"
6. Press the '+' symbol again and select "Basic Types -> String", name it "New String"
7. In the graph itself, press the right mouse button and press "Add", then select "Action -> Blackboard -> Set Variable Value"
8. Drag the "Test" GameObject from the "Blackboard" to the "Variable" slot in the newly created node
9. In the graph itself, press the right mouse button and press "Add", then select "Action -> Animation -> Set Animator Boolean"
10. Drag the "New String" GameObject from the "Blackboard" to the first slot of the "Set Animator Boolean" node
11. Frag the "Test" GameObject from the "Blackboard" to the second slot of the "Set Animator Boolean" node
12. Connect the created "Set Animator Boolean" node with the previously created "Set Variable Value" node by dragging one on top of the other
13. Create a connection (drag an arrow) from the "On Start" node to the combined "Set Variable Value" and "Set Animator Boolean" node
14. In the Hierarchy, press the right mouse button and select "3D Object -> Capsule"
15. Select the "Capsule" GameObject and, in the Inspector, add the "Behavior Agent" Component
16. In the "Behavior Agent" Component, attach the previously created behavior graph ".asset" file to the "Behavior Graph" parameter
17. Enter the Play Mode
18. Observe the Console

Alternative reproduction steps:
1. Open the attached "repro_IN-87592" project
2. Open the "Assets/TestScene.unity" Scene
3. Enter the Play Mode
4. Observe the Console

Expected result: No errors appear
Actual result: "NullReferenceException: Object reference not set to an instance of an object." error appears

Reproducible with: 1.0.0 (6000.0.25f1), 1.0.3 (6000.0.25f1, 6000.1.0a2)

Reproducible on: Windows 10 (22H2), Windows 11 (23H2)
Not reproducible on: No other environments tested

Note: Not able to test the “Behavior“ package on the 2021.3 and 2022.3 as it’s not supported in these streams