Getting FolderNotOpenException while assigning mail subfolders into a treeview item

Question

I would like to put the mail folder structure of an imap account by gmail into a treeview wpf structure and get FolderNotOpenException . Here is the piece of code how I am attempting to achieve this goal:

private void tree_Loaded(object sender, RoutedEventArgs e)
    {
        foreach (AccountData adata in aDatas)
        {
            // ... Create a TreeViewItem.
            TreeViewItem item = new TreeViewItem();
            item.Header = adata.credential.UserName;
            cancel = new System.Threading.CancellationTokenSource();
            System.Threading.CancellationToken token = cancel.Token;
            client = GetIMAPClient(adata.uri,adata.credential,cancel);
            var folders = GetFolders(client, token);
            item.ItemsSource = folders;
            foreach(IMailFolder folder in folders)
            {   
                IEnumerable<IMailFolder> subFolders = folder.GetSubfolders(false,cancel.Token);
                if (subFolders.Count() > 1)
                {
                    foreach (IMailFolder subFolder in subFolders)
                    {
                        subFolder.Open(FolderAccess.ReadOnly);
                        var subItem = new TreeViewItem();
                        //if (subFolder.IsOpen)
                        {
                            subItem.ItemsSource = subFolders;
                            item.Items.Add(subItem);
                        }
                    }
                }
            }
            // ... Get TreeView reference and add both items.
            var tree = sender as TreeView;
            tree.Items.Add(item);
        }
    }

Can someone please help to overcome this problem and to access subfolders ? Thanks