Can't undo or redo actions that add a row to a NSOutlineView

Question

Most of my undo/redo actions have stopped working. I've written a simple example.

aNewNoteFor successfully adds an OutlineItem to its sibling's parent's children and inserts it into the OutlineView row following its sibling.

deleteAChild: is supposed to remove the row and delete it from its parent's children.

But neither undo or redo are enabled.

- (void)aNewNoteFor:(OutlineItem*) sibling
{
    //Ignore note. it's unimportant
    Note* note = [self.document makeDraft:YES];
    OutlineItem* newChild = [[OutlineItem alloc]initWithNote:note];
    NSInteger index = [sibling.parent.children indexOfObject:sibling] +1;
    [sibling.parent.children insertObject:newChild atIndex:index];
    newChild.parent = sibling.parent;
    [_document updateChangeCount:NSChangeDone];
    [_outlineOutlet reloadData];
    [_undoManager registerUndoWithTarget:self selector:@selector(deleteAChild:) object:newChild];
}

-(void)deleteAChild: (OutlineItem*)child
{
    OutlineItem* parent = child.parent;
    [parent.children removeObject:child];
    [_outlineOutlet reloadData];
    [_undoManager registerUndoWithTarget:self selector:@selector(aNewNoteFor:) object:parent];
}
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