Repro:
- download attached project
- keep the VFX Editor docked as is in the project
- open the Portal VFX graph
- switch to scene view
- then go to the UnityLogog folder and open the UnityLogo VFX graph
- notice the console getting spammed by these errors:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.VFX.UI.VFXEditableDataAnchor.OnAttachToPanel (UnityEngine.Experimental.UIElements.AttachToPanelEvent e) (at Library/PackageCache/com.unity.visualeffectgraph@4.6.0-preview/Editor/GraphView/Elements/VFXEditableDataAnchor.cs:126)
UnityEngine.Experimental.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.Experimental.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/Events/EventCallback.cs:62)
UnityEngine.Experimental.UIElements.EventCallbackRegistry.InvokeCallbacks (UnityEngine.Experimental.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/Events/EventCallbackRegistry.cs:376)
UnityEngine.Experimental.UIElements.CallbackEventHandler.HandleEvent (UnityEngine.Experimental.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/Events/EventHandler.cs:112)
UnityEngine.Experimental.UIElements.EventDispatcher.PropagateEvent (UnityEngine.Experimental.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:754)
UnityEngine.Experimental.UIElements.EventDispatcher.ProcessEvent (UnityEngine.Experimental.UIElements.EventBase evt, UnityEngine.Experimental.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:639)
UnityEngine.Experimental.UIElements.EventDispatcher.ProcessEventQueue () (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:386)
UnityEngine.Experimental.UIElements.EventDispatcher.OpenGate () (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:351)
UnityEngine.Experimental.UIElements.EventDispatcher+Gate.Dispose () (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:75)
UnityEngine.Experimental.UIElements.EventDispatcher.ProcessEvent (UnityEngine.Experimental.UIElements.EventBase evt, UnityEngine.Experimental.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:412)
UnityEngine.Experimental.UIElements.EventDispatcher.Dispatch (UnityEngine.Experimental.UIElements.EventBase evt, UnityEngine.Experimental.UIElements.IPanel panel, UnityEngine.Experimental.UIElements.DispatchMode dispatchMode) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:307)
UnityEngine.Experimental.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.Experimental.UIElements.EventBase e, UnityEngine.Experimental.UIElements.DispatchMode dispatchMode) (at C:/buildslave/unity/build/Modules/UIElements/Panel.cs:176)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (UnityEngine.Experimental.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:245)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:68)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)

Note: this issue MAY affect 2019.1 and 2019.2, but because of this bug: https://fogbugz.unity3d.com/f/cases/1116430 i cannot verify this. I will check once this other issue gets fixed!