Textbox not getting focused

Question

I have a MainWindow with a ViewModel, when I press a button, a new SubWindow, with its onw ViewModel is opened, which recieves the "parent" ViewModel, like this:

    private AppViewModel vm;

    public void Show(AppViewModel vm)
    {
        this.vm = vm;
        this.DialogShown = true;
    }

When I close the SubWindow this method is called:

    public void Hide()
    {
        vm.IsFocused = true;
        this.DialogShown = false;
    }

But in my MainWindow my TextBox doesn't get focused...

This is my property:

 public bool IsFocused
    {
        get { return _isFocused; }
        set
        {
            if (value != _isFocused)
            {
                _isFocused = value;
                OnPropertyChanged("IsFocused");
            }
        }
    }

And this is my xaml:

<TextBox Focusable="True" AcceptsReturn="True" ext:FocusExtension.IsFocused="{Binding IsFocused, Mode=TwoWay}"  IsTabStop="False"/>

What am I doing wrong?