How to reproduce:
1. Create a new HDRP project
5. Create a new VFX and open it
6. Add Heads and Trails system
7. Select Set Lifetime Random block in the Initialize context
8. Right-click mouse and convert it to a Subgraph block

Expected result: Subgraph block is created and no errors are thrown to the Console
Actual result: Subgraph block is not created and 'NullReferenceException: Object reference not set to an instance of an object' error is thrown to the Console

Reproducible with: 2020.2.5f1(10.3.1, 10x.x/release),
Not reproducible with: 2019.4.17f1(7.3.1, 7.5.3), 2021.1.0b4(11.0.0), 2021.2.0a6(12.0.0)

Notes:
- Error message: NullReferenceException: Object reference not set to an instance of an object
UnityEditor.VFX.UI.VFXPaste.RegisterContexts (UnityEditor.VFX.UI.VFXViewController viewController) (at Library/PackageCache/com.unity.visualeffectgraph@10.3.1/Editor/GraphView/Views/VFXPaste.cs:642)
UnityEditor.VFX.UI.VFXPaste.PasteAll (UnityEditor.VFX.UI.VFXViewController viewController, UnityEngine.Vector2 center, UnityEditor.VFX.UI.VFXCopyPasteCommon+SerializableGraph& serializableGraph, UnityEditor.VFX.UI.VFXView view, UnityEditor.VFX.UI.VFXGroupNodeController groupNode, System.Collections.Generic.List`1[T] nodesInTheSameOrder) (at Library/PackageCache/com.unity.visualeffectgraph@10.3.1/Editor/GraphView/Views/VFXPaste.cs:192)
UnityEditor.VFX.UI.VFXPaste.DoPaste (UnityEditor.VFX.UI.VFXViewController viewController, UnityEngine.Vector2 center, System.Object data, UnityEditor.VFX.UI.VFXView view, UnityEditor.VFX.UI.VFXGroupNodeController groupNode, System.Collections.Generic.List`1[T] nodesInTheSameOrder) (at Library/PackageCache/com.unity.visualeffectgraph@10.3.1/Editor/GraphView/Views/VFXPaste.cs:63)