NullReferenceException when importing Graph that has subgraph node

Steps to reproduce:

1. Create a custom Graph type that supports subgraphs

   [Graph(extension:"abc", GraphOptions.SupportsSubgraphs)]
   public class GraphTest : Graph
   Unknown macro: {     [MenuItem("Assets/Create/Graph/GraphTest")]     public static void CreateGraph()     Unknown macro}
   }

2. Create two assets of your Graph type using the project window
3. Double click one of the graphs to open a graph window
4. Drag and drop the second graph asset into the graph window to create a subgraph node
5. Save the graph and close the window (and any other GTK related windows)
6. Re-import a script in your project to trigger a domain reload
7. Observe the console

Reproducible with versions: 6000.6.0a1

Not reproducible with 6000.5.0a8, 6000.4.0b11

Tested on (OS): MacOS