How to reproduce:

1. Open attached project
2. Open scene Main.unity
3. Select the Undo Test gameobject
4. Click the Modify Children button
- The int field in the gameobject, as well as all of the child objects, is modified
5. Press the Undo Changes button
- Notice the exception thrown

- There are a few more issues present (they are all also described in the UndoTestEditor.cs script):

- None of these register the children for undoing:
- Undo.RegisterCompleteObjectUndo(Target, "Modify Children");
- Undo.RegisterFullObjectHierarchyUndo(Target, "Modify Children");
- Undo.RecordObject(Target, "Modify Children");

- This crashes the Editor upon undoing:
- Undo.RegisterCompleteObjectUndo(Target.gameObject, "Modify Children");

- This doesn't register components:
- Undo.RecordObject(Target.gameObject, "Modify Children");