CustomEditor for a list of polymorphic items

Hi all, been struggling with this for a while so I hope someone can help.

To schedule cutscenes in my game I have a scriptableobject named Scene:

 public class Scene: ScriptableObject
 {
     public List<ActingDirection> directions = new List<ActingDirection>();
 }

Scene has a list of ActingDirections:

 public class ActingDirection {
     public int actor;   //Who is performing the action
     public float timeStart;   //The time after the previous action this one should play
 }

And there are two classes that extend ActingDirection: ActingDialogue and ActingAnimation, which respectively contain a string of dialogue, or an enum specifying an animation.

I've managed to get a custom editor working for the Dialogues on their own:

 list = new ReorderableList(serializedObject,
                 serializedObject.FindProperty("directions"),
                 true, true, true, true);
 
         list.drawElementCallback =
         (Rect rect, int index, bool isActive, bool isFocused) => {
             var element = list.serializedProperty.GetArrayElementAtIndex(index);
             rect.y += 2;
             EditorGUI.PropertyField(
                 new Rect(rect.x, rect.y, 60, EditorGUIUtility.singleLineHeight),
                 element.FindPropertyRelative("actor"), GUIContent.none);
             EditorGUI.PropertyField(
                 new Rect(rect.x + 60, rect.y, 600, EditorGUIUtility.singleLineHeight),
                 element.FindPropertyRelative("dialogue"), GUIContent.none);
             EditorGUI.PropertyField(
                 new Rect(rect.x + rect.width - 55, rect.y, 40, EditorGUIUtility.singleLineHeight),
                 element.FindPropertyRelative("timeStart"), GUIContent.none);
             EditorGUI.PropertyField(
                 new Rect(rect.x + rect.width - 10, rect.y, 20, EditorGUIUtility.singleLineHeight)

That worked fine for just the dialogue objects but when I try to combine both the list shows up blank and complains about an object reference not set to an instance.

I'd ideally like something like the mockup below:

Is this possible? I don't know enough about how serialization or editor scripting works to figure this one out on my own!