*Steps to reproduce:*
# 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)
# Open the 'New Shader Graph.shadergraph' asset in the Asset Browser
# Drag the legacy 'Color' property from the blackboard into the graph
# Click on the Color property in the graph (not in the Blackboard) and click 'Update' in the Graph Inspector
# Undo the update with Ctrl + Z
# 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.