尝试以下操作：

class My_Widget extends WP_Widget {

    /**
     * Sets up the widgets name etc
     */
    public function __construct() {
        $widget_ops = array(
            \'classname\' => \'my_widget\',
            \'description\' => \'My Widget is awesome\',
        );
        parent::__construct(\'my_widget\', \'My Widget\', $widget_ops);
    }

    /**
     * Outputs the content of the widget
     *
     * @param array $args
     * @param array $instance
     */
    public function widget($args, $instance) {
        echo $args[\'before_widget\'];
        echo $instance[\'title\'];
        echo $args[\'after_widget\'];
    }

    /**
     * Outputs the options form on admin
     *
     * @param array $instance The widget options
     */
    public function form($instance) {

        $title = !empty($instance[\'title\']) ? $instance[\'title\'] : esc_html__(\'New title\', \'text_domain\');
        if(isset($instance[\'error\'])){
            echo $instance[\'error\'];
        }
        ?>
            <p>
            <label for="<?php echo esc_attr($this->get_field_id(\'title\')); ?>"><?php esc_attr_e(\'Title:\', \'text_domain\'); ?></label> 
            <input class="widefat" id="<?php echo esc_attr($this->get_field_id(\'title\')); ?>" name="<?php echo esc_attr($this->get_field_name(\'title\')); ?>" type="text" value="<?php echo esc_attr($title); ?>">
            </p>
        <?php
    }

    /**
     * Processing widget options on save
     *
     * @param array $new_instance The new options
     * @param array $old_instance The previous options
     */
    public function update($new_instance, $old_instance) {

        // !!!!!!!
        if (empty($new_instance[\'title\'])) {
            $instance[\'error\'] = "ERROR!! title empty";
        }else{
            $instance[\'title\'] = strip_tags( $new_instance[\'title\'] );
        }
        return $instance;
    }

}