function _updateUI() {
        if (DocumentManager.getCurrentDocument() !== null) {
            if (!CommandManager.get(Commands.VIEW_INCREASE_FONT_SIZE).getEnabled()) {
                // If one is disabled then they all are disabled, so enable them all
                CommandManager.get(Commands.VIEW_INCREASE_FONT_SIZE).setEnabled(true);
                CommandManager.get(Commands.VIEW_DECREASE_FONT_SIZE).setEnabled(true);
                CommandManager.get(Commands.VIEW_RESTORE_FONT_SIZE).setEnabled(true);
            }
        } else {
            // No current document so disable all of the Font Size commands
            CommandManager.get(Commands.VIEW_INCREASE_FONT_SIZE).setEnabled(false);
            CommandManager.get(Commands.VIEW_DECREASE_FONT_SIZE).setEnabled(false);
            CommandManager.get(Commands.VIEW_RESTORE_FONT_SIZE).setEnabled(false);
        }
    }
@private
Updates the user interface appropriately based on whether or not a document is
currently open in the editor.