update tab control wpf from another class

good morning
 
I have two class
1-class Window1 : that contains the tab control added
2-class DesignerCanvas :must add a text in tab control
I then add the following code in the class DesignerCanvas

   public  Window1 form; 
         public  DesignerCanvas(Window1 form)
    {
        this.form = form;
    }
	  public void SendBackward_Executed( object sender, ExecutedRoutedEventArgs e)
        {
            ((Window1)form).tabItem1.Content = "new text"  ;
        }

 
In class window1 I add the following code

 private DesignerCanvas test = new DesignerCanvas();

and in the method public window1 I add:

 public Window1()
        {
            InitializeComponent();
            test.form = this;
          //here is the error
            test.SendBackward_Executed(object sender , System.Windows.Input.ExecutedRoutedEventArgs e);
            
        }

is there any other way to call the method??