嗨，为什么不使用多个复选框而不是选择选项字段呢。此外，您还使用了不推荐使用的函数，如“create\\u function”。我修改了您的代码，将选择选项替换为多个复选框选择。

function myCatWidget() {
    register_widget( \'my_custom_cat_widget\' );
}
add_action( \'widgets_init\', \'myCatWidget\' );

class my_custom_cat_widget extends WP_Widget {

   function __construct() { 
        parent::__construct(\'my_custom_cat_widget\', __(\'My cat widget\', \'wpb_widget_domain\'), 
            array( \'description\' => __( \'Your description\', \'wpb_widget_domain\' ), ) 
        );
    }

public function form( $instance ) {
    $title = isset($instance[ \'title\' ]) ? $instance[ \'title\' ] : \'Categories\';
    $instance[\'postCats\'] = !empty($instance[\'postCats\']) ? explode(",",$instance[\'postCats\']) : array();
    ?>

    <p>
        <label for="<?php echo $this->get_field_id( \'title\' ); ?>">Title</label>
        <input type="text" class="widfat" id="<?php echo $this->get_field_id( \'title\' ); ?>" name="<?php echo $this->get_field_name( \'title\' ); ?>" style="width: 100%;" value="<?php echo $title; ?>"/>
    </p>

    <p>
        <label for="<?php echo $this->get_field_id( \'postCats\' ); ?>"><?php _e( \'Select Categories you want to show:\' ); ?></label><br />
        <?php $args = array(
                \'post_type\' => \'post\',
                \'taxonomy\' => \'category\',
            );
            $terms = get_terms( $args );
        //print_r($terms);
         foreach( $terms as $id => $name ) { 
            $checked = "";
            if(in_array($name->name,$instance[\'postCats\'])){
                $checked = "checked=\'checked\'";
            }
        ?>
            <input type="checkbox" class="checkbox" id="<?php echo $this->get_field_id(\'postCats\'); ?>" name="<?php echo $this->get_field_name(\'postCats[]\'); ?>" value="<?php echo $name->name; ?>"  <?php echo $checked; ?>/>
            <label for="<?php echo $this->get_field_id(\'postCats\'); ?>"><?php echo $name->name; ?></label><br />
        <?php } ?>
    </p>

    <?php

}

public function update( $new_instance, $old_instance ) {
    $instance = $old_instance;

    $instance[ \'title\' ] = strip_tags( $new_instance[ \'title\' ] );
    $instance[\'postCats\'] = !empty($new_instance[\'postCats\']) ? implode(",",$new_instance[\'postCats\']) : 0;
    return $instance;
}

public function widget( $args, $instance ) {
    extract( $args );

    $title = apply_filters( \'widget_title\', $instance[ \'title\' ] );
    $postCats = $instance[ \'postCats\' ];
    $categories_list = explode(",", $postCats);

    echo $before_widget;

    if( $title ) {
        echo $before_title . $title . $after_title;
    }

    $args = array(\'post_type\' => \'post\',\'taxonomy\' => \'category\',);
    $terms = get_terms( $args );
    ?>
    <ul>
        <?php
            foreach ($categories_list as $cat) {
                foreach($terms as $term) {
                    if($cat === $term->name) {
                        echo "<li>".$term->name."</li>";                                    
                    }
                }
            }
        ?>
    </ul>
    <?php
    echo $after_widget;
}
}