How to do a custom node graph editor in Unity 2018.4 or 2019.2

DialogueGraphEditor.cs

// The commented out parts are for Unity 2018.4
//using System.Reflection;
//using UnityEditor.Experimental.UIElements.GraphView;
//using UnityEngine.Experimental.UIElements;
//using UnityEngine.Experimental.UIElements.StyleEnums;
using System.Collections.Generic;
using System.Linq;
using UnityEditor;
using UnityEditor.Experimental.GraphView;
using UnityEngine;
using UnityEngine.UIElements;

public class DialogueEditorWindow : EditorWindow
{
    private bool initialized = false;
    private DialogueGraphView view;

    [MenuItem("Window/Dialogue Editor")]
    public static void ShowWindow()
    {
        var window = EditorWindow.GetWindow<DialogueEditorWindow>("Dialogue Editor");

        if (!window.initialized)
        {
            window.Initialize();
            Debug.Log("Dialogue Editor initialized");
            window.initialized = true;
        }

        window.Show();
    }

    private void Initialize()
    {
        view = new DialogueGraphView();
        view.style.alignSelf = Align.Stretch;
        view.style.flexGrow = 1;

        view.AddManipulator(new ContentDragger());
        view.AddManipulator(new SelectionDragger());
        view.AddManipulator(new RectangleSelector());
        view.AddManipulator(new ClickSelector());
        view.SetupZoom(0.5f, 2f);

        rootVisualElement.Add(view);
    }

    /*private VisualElement rootVisualElement
    {
        get
        {
            return GetType()
                .GetProperty("rootVisualContainer", BindingFlags.Instance | BindingFlags.NonPublic)
                .GetValue(this) as VisualElement;
        }
    }*/
}

public sealed class DialogueGraphView : GraphView
{
    public override List<Port> GetCompatiblePorts(Port startPort, NodeAdapter nodeAdapter)
    {
        return ports.ToList(); // All ports are compatible!
    }

    public override void BuildContextualMenu(ContextualMenuPopulateEvent evt)
    {
        base.BuildContextualMenu(evt);
        //evt.menu.AppendAction("Create", OnCreate, a => DropdownMenu.MenuAction.StatusFlags.Normal);
        evt.menu.AppendAction("Create", OnCreate);
    }

    private void OnCreate(DropdownMenuAction action)
    {
        var node = new Node();
        node.title = "NODE";

        var port1 = node.InstantiatePort(Orientation.Horizontal, Direction.Input, Port.Capacity.Single, typeof(string));
        port1.portName = "in";
        port1.source = "";
        node.inputContainer.Add(port1);

        var port2 = node.InstantiatePort(Orientation.Horizontal, Direction.Output, Port.Capacity.Multi, typeof(string));
        port2.portName = "out";
        port2.source = "";
        node.outputContainer.Add(port2);

        node.SetPosition(new Rect(action.eventInfo.localMousePosition, Vector2.zero));
        AddElement(node);
    }
}