How to reproduce:
1. Create a new project
2. Install the Shader Graph package
3. Create a new Sub Graph
4. In the Shader Editor disable the Blackboard
5. Create a new node
6. In the top left corner of the Shader Editor select 'Save Asset'
7. Close the Shader Editor
8. In the Project window double-click on the Sub Graph
9. Try to duplicate the created node (either by right-clicking and selecting Duplicate or using Ctrl+D shortcut)
10. Observe the Console

Expected result: The node is duplicated, no visible errors present
Actual result: The node is not duplicated, 'ArgumentNullException: Value cannot be null.' appears in the Console

Reproducible with: Shader Graph 9.0.0-preview.34 (2020.1.17f1, 2020.2.1f1), 10.2.2 (2020.2.1f1, 2021.1.0b1), 11.0.0 (2021.1.0b1)
Not reproducible with: Shader Graph 9.0.0-preview.14 (2020.1.17f1, 2020.2.1f1)
Cannot test with: Shader Graph 9.0.0-preview.14, 9.0.0-preview.34 (2018.4.30f1, 2019.4.17f1), 10.2.2 (2018.4.30f1, 2019.4.17f1, 2020.1.17f1) (the Shader Graph package versions are not supported), 9.0.0-preview.14, 9.0.0-preview.34 (2021.1.0b1) (due to errors)

Notes:
-The issue is also reproducible when trying to copy and paste the node
-The issue is reproducible only after reopening the Graph
-The issue does not reproduce once you open the Blackboard