can’t see all pages in dashboard (can only see 3) /wp-admin/edit.php > 500 error

1. Please open your wp-config.php file and change the parameter like this:

   define('WP_DEBUG', true);
   

Once this parameter is activated, you can see more details about the error.

2. Try to activate any default WordPress theme like TwentySeventeen and look if the error still exists.

Update:

function avia_backend_compatibility_custom_field_filter($custom_fields, $post_id)
{
    if(empty($custom_fields))
    {
        $custom_fields = array(
             'slideshow' => array(
                 array( 'slideshow_image' => '' )
             )
        );
    }

    if(isset($custom_fields['slideshow']) && is_array($custom_fields['slideshow']) && isset($custom_fields['slideshow'][0]['slideshow_image']))
    {
        $post_thumbnail_id = get_post_meta( $post_id, '_thumbnail_id', true );

        if($custom_fields['slideshow'][0]['slideshow_image'] == "" && $post_thumbnail_id)
        {
            $custom_fields['slideshow'][0]['slideshow_image'] = $post_thumbnail_id;
        }
    }
    return $custom_fields;
}