Repro steps:
1. Create a new VFX asset and open it
2. Create a node (i.e. Add), right click on it and convert it to a subgraph operator
3. Open the newly-created subgraph
4. Add an output to blackboard
5. Try to open the original VFX
6. Unity doesn't recognize the type / a console error appears

Expected outcome:
- no errors, the VFX graph can be opened

Notes:
- Must have regressed relatively recently
- Console error:
Exception thrown while invoking [OnOpenAssetAttribute] method 'VisualEffectAssetEditor:OnOpenVFX (int,int)' : Exception: Numbers of output expressions (1) does not match number of output (with expression)s slots (0)
UnityEditor.VFX.VFXOperator.UpdateOutputExpressions () (at Packages/com.unity.visualeffectgraph-7.1.6/Editor/Models/Operators/VFXOperator.cs:125)