How to change CommandTarget

Question

I have class DesignerCanvas where I have RoutedCommand.

public class DesignerCanvas
{
    public static RoutedCommand SelectAll = new RoutedCommand();

    public DesignerCanvas()
    {
        this.CommandBindings.Add(new CommandBinding(DesignerCanvas.SelectAll, SelectAll_Executed));
        SelectAll.InputGestures.Add(new KeyGesture(Key.A, ModifierKeys.Control));
    }
}

In ResourcesDictionary I've got MyToolbar in this I've got buttons that have specify commands like this:

<Button Margin="3" Width="55" Style="{StaticResource ToolBarButtonBaseStyle}"
            HorizontalContentAlignment="Center"
            Command="{x:Static DesignerCanvas.SelectAll}"
            CommandTarget="{Binding ElementName=MyDesigner} /">

And everything works perfect unless I don't close main window and than create it ones again with new MainWindow().Show(). All commands want to execute in old DesignerCanvas (from the original MainWindow) What can I do to change CommandTarget to new DesignerCanvas witch was created in new MainWindow?

EDIT

MyDesigner in MainWindow:

<s:DesignerCanvas
   Focusable="true"
   x:Name="MyDesigner"
   Background="{StaticResource WindowBackgroundBrush}"
   ContextMenu="{StaticResource DesignerCanvasContextMenu}"
   SnapsToDevicePixels="True"
   Margin="-2" />