To reproduce:

1. Open project, open editor Custom -> Open
This editor has 4 buttons:
- Create Trackers
- Create Method 1
- Create Method 2
- Create Method 3

The create trackers, simply creates parent objects for the 3 methods (it gets called when the editor is opened too).

2. Select Tracker Method 1
3. Open up it's list in the inspector so you can view it's elements
4. Click Create Method 1, notice the child added in the hierarchy and now the list has 1 element
5. Cmd+z to undo, child it now gone, list has 0 elements
6. Cmd+shift+z to redo, child is back, list has 1 element but the element is null

Now repeat steps 2-6 with the other method objects / buttons.

What you'll see:
- With Method 1: The list has a null element
- With Method 2: It works
- With Method 3: When you undo, the script disappears from the tracker object (!!)

Not sure if the Method 3 API is even something that should be used for that or not, but what I assume is that it would call RegisterCompleteObjectUndo on it and all it's children.