Clicking on "Cancel" button from "Create Subgraph" pop-up causes NullReferenceException, refer attached video.

Steps to repro:
1. Create a new project
2. Windows > Package Manager > Visual Effects Graph > Install
3. Project > Packages > Visual Effect Graph > Editor > Templates > Open Default Subgraph Block
4. Focus on "Block Subgraph"
5. Right-click > Block Subgraph > Convert To Subgraph
6. Click on the "Cancel" button from "Create Subgraph" Window

Actual Result:
NullReferenceException is thrown click on "Cancel" button from "Create Subgraph" Window while converting Subgraph

Expected Result:
"Create Subgraph" pop-up cancels without any exception

RReproducible in:
2020.1.0a6, 2019.3.0b5, 2019.3.0b4

Working fine on:
2019.3.0b3, 2019.3.0a10

Package version found: 7.1.2, 7.1.1

Package version working fine: 7.0.1, 7.0.0

NOTE:
Visual Effect Graph is introduced in 2019.3.0a10.

Environment:
Windows 10 & macOS 10.14

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.VFX.UI.VFXViewController.GetController (UnityEditor.VFX.VisualEffectResource resource, System.Boolean forceUpdate) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.2/Editor/GraphView/Views/Controller/VFXViewController.cs:1280)
UnityEditor.VFX.UI.VFXConvertSubgraph+Context.CreateUniqueSubgraph (System.String typeName, System.String extension, System.Func`2[T,TResult] createFunc) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.2/Editor/GraphView/Views/VFXConvertSubgraph.cs:345)
UnityEditor.VFX.UI.VFXConvertSubgraph+Context.ConvertToSubgraphContext (UnityEditor.VFX.UI.VFXView sourceView, System.Collections.Generic.IEnumerable`1[T] controllers, UnityEngine.Rect rect) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.2/Editor/GraphView/Views/VFXConvertSubgraph.cs:219)
UnityEditor.VFX.UI.VFXConvertSubgraph.ConvertToSubgraphContext (UnityEditor.VFX.UI.VFXView sourceView, System.Collections.Generic.IEnumerable`1[T] controllers, UnityEngine.Rect rect) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.2/Editor/GraphView/Views/VFXConvertSubgraph.cs:19)
UnityEditor.VFX.UI.VFXView.ToSubgraphContext (UnityEngine.UIElements.DropdownMenuAction a) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.2/Editor/GraphView/Views/VFXView.cs:1760)
UnityEngine.UIElements.DropdownMenuAction.Execute () (at <afbdfd007ccb47e98566943b0715a4f7>:0)
UnityEditor.UIElements.EditorMenuExtensions+<>c__DisplayClass0_0.<PrepareMenu>b__0 () (at <52c46852ed4749ed926125cda2005635>:0)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, System.Int32 selected) (at <52c46852ed4749ed926125cda2005635>:0)