Undo.RecordObjects isn't working for objects referencing each other when one gets deleted.

To reproduce:
1. Open the attached project
2. Click "Custom -> Open Window" menu item to bring up a custom editor window
3. Click "Add Tracker"
4. Click "Add Child" twice
5. Expand the hierarchy of the tracker, check that their variables are set properly, same for the script objects inside it
6. Click "Delete Child", notice how one of them will be gone
7. Undo
8. Inspect the scripts and notice the null references

To see how it should work, before repeating the steps, open up the editor window script and replace the RecordObjects for the RegisterCompleteObjectUndo. Notice that at the end, the references are all properly set.