"NullReferenceException" is thrown when converting an empty Local Subgraph to an Asset Subgraph

Steps to reproduce:

1. Open the attached project
2. Go to Assets > Create > Graph Toolkit Samples > Texture Maker Graph
3. Open the created graph
4. Right-click on any empty space
5. Select "Create Empty Local Subgraph"
6. Right-click on the subgraph and select "Convert to Asset Subgraph"
7. Observe the Console

Actual results: "NullReferenceException: Object reference not set to an instance of an object" error is thrown

Expected results: No errors

Reproducible with versions: 6000.5.0a2, 6000.5.0b4, 6000.6.0a2
Not reproducible with versions: 6000.4.3f1, 6000.5.0a1

Tested on (OS): macOS 26.3, Ubuntu 24.04

Notes:

• The asset still gets created correctly