Reproduction steps:
1. Open the user's attached project
2. In the Project window, open Test.shadergraph
3. Select Custom Function node and open Graph Inspector
4. In the Node Settings, select and delete the Source file

Expected result: Source file is deleted
Actual result: Custom Function node is deleted

Reproducible with: 7.5.1 - 7.5.3 (2019.4.20f1), 10.3.1 (2020.2.4f1), 11.0.0 (2021.1.0b6), 12.0.0 (2021.2.0a4)
Not reproducible with: 7.3.1 - 7.4.3 (2019.4.20f1)
Could not test with: 2018.4.32f1 (could not downgrade)