Repro steps:
1. In a VFX graph, right click and create an Empty Subgraph Operator.
2. Right-click the newly created subgraph and select 'Enter Subgraph'
3. A null ref exception is thrown

Notes:
- A cosmetic issue, but we should probably handle it by either removing the 'Enter Subgraph' option if no subgraph is assigned or changing the exception to a warning instructing the user to assign a subgraph in the inspector.
- Happens for both empty subgraph operators and blocks