Reproduction steps:
1. Open the attached project "My Project"
2. Open Shader Graph "Assets/Example/4D Noise"

Expected result: No errors are thrown
Actual result: Subgraphs that use Dropdown Nodes throw errors

Reproducible with: 12.0.0 (2021.2.0b15), 12.1.0 (2021.2.1f1)
Not reproducible with: 12.0.0 (2021.2.0b14, 2022.1.0a1), 13.1.0 (2022.1.0a14)
Could not test with: 7.7.1 (2019.4.32f1), 10.7.0 (2020.3.22f1), 11.0.0 (2021.1.28f1) (no Dropdown Node type)

Notes:
- removing Dropdown Node from an affected Subgraph makes it functional again
- reproducible both with URP and HDRP