MyWindow.cs

using UnityEditor;
using UnityEngine;

public class MyWindow : EditorWindow
{
    public MySO testNode;

    [MenuItem("TEST/OPEN WINDOW")]
    static void Init()
    {
        MyWindow window = (MyWindow)EditorWindow.GetWindow(typeof(MyWindow));
        window.Show();
    }

    private SerializedObject _testNodeSo;

    void OnGUI()
    {
        testNode = EditorGUILayout.ObjectField(testNode, typeof(MySO), false) as MySO;
        if (testNode == null)
        {
            EditorGUILayout.LabelField("Please create a 'MySO' and assign it");
        }
        else
        {
            if (_testNodeSo == null)
            {
                _testNodeSo = new SerializedObject(testNode);
            }
            // Floats work as expected
            EditorGUILayout.PropertyField(_testNodeSo.FindProperty("myFloat"));
            // Events are wonky and do not update except on method change
            EditorGUILayout.PropertyField(_testNodeSo.FindProperty("myEvent"));

            if (GUI.changed)
            {
                _testNodeSo.ApplyModifiedProperties();
            }
        }
    }
}