Legacy nodes in the graph can result in a NullReferenceException when updating node

Steps to reproduce:

1. Open the attached URP Unity Project in your modern Unity version (original project version is 2019.3.15f1 so that it has a legacy property)
2. Open the 'New Shader Graph.shadergraph' asset in the Asset Browser
3. Drag the legacy 'Color' property from the blackboard into the graph
4. Click on the Color property in the graph (not in the Blackboard) and click 'Update' in the Graph Inspector
5. Undo the update with Ctrl + Z
6. Click on the Color property in the graph again and click 'Update' in the Graph Inspector

Actual results: A NullReferenceException is logged to console with the message 'Object reference not set to an instance of an object'

Expected results: The Color property is updated without a NullReferenceException

Reproducible with versions: 

• 6000.0.12f1
• 2022.3.39f1
• 2021.3.40f1

Not reproducible with versions: -

Can’t test with versions: -

Tested on (OS): Windows 11

Notes:

• Updating the Color property from the Blackboard will not cause this error. It is specifically when updating the property from the graph after undoing a previous Update with Ctrl + Z.