Dynamically load TabItem - wpf

Question

I have 2 xaml files: MainWindow.xaml and Subscreen.xaml.
I want to dynamically load Subscreen.xaml as a TabItem under TabControl of MainWindow.xaml.
There are a ViewModel for each View.
MainWindow.xaml: MainViewModel
SubScreen.xaml: SubViewModel
The following is the test code:
[TestMethod]
public void MyTest2()
{
// Arrange
// Initilize a subVM of CMSEditorViewModel type
var subVM = new SubViewModel();
// Initialize a mainVM of CMSEditorMainViewModel type
var mainVM = new MainViewModel();
// Initialize a MainWindow of DynamicCMS.Exe.CMSEditor
var mainWindow = new MyEditor.MainWindow();
mainWindow.DataContext = mainVM;
ContentPresenter presenter = new ContentPresenter();
using (var stream = System.IO.File.OpenRead(CmsPath.DirViewWithBS + "Subscreen.xaml"))
{
DataTemplate template = XamlReader.Load(stream) as DataTemplate;
presenter.ContentTemplate = template;
presenter.Content = subVM;
}
// Create a TabItem of TabControl
TabItem item = new TabItem();
item.Header = "Tab1";
item.Content = presenter;
item.Name = "tab1";
// Get "mainTabControl" TabControl from MainWindow
CustomTabControl tab = CmsUtil.GetControl((Visual)mainWindow.Content, "mainTabControl") as CustomTabControl;
// Add TabItem to TabControl
tab.Items.Add(item);
// Act
CustomTabControl customTabControlEditor = (CustomTabControl)CmsUtil.GetControl((Visual)mainWindow.Content, "EditTabControl");
// Assert
Assert.IsNotNull(customTabControlEditor);
}
The "mainTabControl" is defined in MainWindow.xaml -> I can get this "mainTabControl" TabControl.
The "EditTabControl" is defined in Subscreen.xaml -> I can't get this "EditTabControl" TabControl -> customTabControlEditor is null.
Is anything wrong in the code that loading dynamically?
Thanks in advance.