NullReferenceException throws on clicking "Play", "Stop" & "Custom" button in the "VFX Event Tester" when "Event Attributes" added in the list and "Asset Template" is set to None in the inspector under Visual Effect component, refer attached video.

Steps to repro:
1. Create a new project
2. Windows > Package Manager > Visual Effects Graph > Install
3. Hierarchy > Create > Visual Effects > Visual Effect
4. Select Visual Effect GameObject in the Hierarchy
5. Scene > VFX Event Tester > Add Event Attribute in the list
5. Click on "Custom" button

Actual Result
NullReferenceException throw on selecting "Play", "Stop", "Custom" button

Expected Result:
Exceptions don't throw on selecting "Play", "Stop", "Custom" button in the "VFX Event Tester"

NOTE:
Visual Effect Graph is introduced in 2019.3.0a10 With VFX Graph version 7.0.0

Tested with Unity version
2020.1.0a19, 2019.3.0f6(69ba09c9b97e), 2019.3.0a10.

Package found version: 7.1.7, 7.1.6, 7.1.5, 7.1.1, 7.0.0

Environment:
Windows 10 & macOS 10.14

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Experimental.VFX.Utility.VFXEventTesterWindow.SendEvent (System.String name) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.7/Editor/Utilities/EventTester/VFXEventTesterWindow.cs:290)
UnityEditor.Experimental.VFX.Utility.VFXEventTesterWindow.WindowGUI () (at Library/PackageCache/com.unity.visualeffectgraph@7.1.7/Editor/Utilities/EventTester/VFXEventTesterWindow.cs:269)
UnityEditor.Experimental.VFX.Utility.VFXEventTesterWindow.WindowFunction (UnityEngine.Object target, UnityEditor.SceneView sceneView) (at Library/PackageCache/com.unity.visualeffectgraph@7.1.7/Editor/Utilities/EventTester/VFXEventTesterWindow.cs:146)
UnityEditor.SceneViewOverlay.WindowTrampoline (System.Int32 id) (at <b9bb103d86cd4249b6a1c8b6aa2850c7>:0)
UnityEngine.GUILayout+LayoutedWindow.DoWindow (System.Int32 windowID) (at <1e9a978af2924f25af72a3b481c8261f>:0)
UnityEngine.GUI.CallWindowDelegate (UnityEngine.GUI+WindowFunction func, System.Int32 id, System.Int32 instanceID, UnityEngine.GUISkin _skin, System.Int32 forceRect, System.Single width, System.Single height, UnityEngine.GUIStyle style) (at <1e9a978af2924f25af72a3b481c8261f>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)