property savedFocusedBox

This property returns the previous focused box. This is the focus box before menu or window inactivation.

function onUndoFromMenuBar()
{
    var savedFocusedBox = scene.savedFocusedBox;
    if (savedFocusedBox != null && savedFocusedBox.boxType == "nativeText")
    {
        savedFocusedBox.undo();
    }
}