NullReferenceException error is thrown on renaming by right click on the property parameter.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Visual Effects Graph > Install.
3. Packages > Visual Effect Graph > Editor > Templates > Open Default Subgraph Block.
4. Add Vector2 property > Expand.
5. Right-click on Value > Rename.

Actual Result:
NullReferenceException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2019.3.0a12, 2019.3.0a10.

NOTE:
1. Visual Effects is introduced in 2019.3.0a10.
2. Could not test on Mac due to https://fogbugz.unity3d.com/f/cases/1174796/

Environment:
Only on Windows.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.VFX.UI.VFXBlackboardCategory.OpenTextEditor () (at library/packagecache/com.unity.visualeffectgraph@7.0.1/editor/graphview/blackboard/vfxblackboardcategory.cs:332)
UnityEditor.VFX.UI.VFXBlackboardCategory.<BuildContextualMenu>b__36_0 (UnityEngine.UIElements.DropdownMenuAction a) (at library/packagecache/com.unity.visualeffectgraph@7.0.1/editor/graphview/blackboard/vfxblackboardcategory.cs:342)
UnityEngine.UIElements.DropdownMenuAction.Execute () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEditor.UIElements.EditorMenuExtensions+<>c__DisplayClass0_0.<PrepareMenu>b__0 () (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, System.Int32 selected) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)