one of function code conflict with this plugins

it was totally full oneday i took find out my problem. my code was work before nicely but after i install this plugins. my code not work. i had no idea what wrong my code.. then lot of RnD i did then found this is for this plugins… my code is custom gallery. bellow is my code.. any idea why and how to fix it ?

function custom_gallery_shortcode( $attr = array(), $content = '' )

{

        $attr['itemtag']        = "li";
        $attr['icontag']        = "";
        $attr['captiontag']     = "p";

        // Run the native gallery shortcode callback:
        $html = gallery_shortcode( $attr );
        // Remove all tags except a, img,li, p
        $html = strip_tags( $html, '<a><img><li><p>' );
        // Some trivial replacements:
        $from = array(  

            "class='gallery-item'",
            "class='gallery-icon landscape'",
            'class="attachment-thumbnail"',
            'a href=', 

        );              

        $to = array( 

            '',

            '',

            '', 

            'a class="lightbox" href=', 

        );

        $html = str_replace( $from, $to, $html );

        // Remove width/height attributes:

        $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
        // Wrap the output in ul tags:

        $html = sprintf( '<ul class="gallery">%s</ul>', $html );

        return $html;

}

add_shortcode( 'gallery', 'custom_gallery_shortcode' );
add_filter( 'use_default_gallery_style', '__return_false' );

https://wordpress.org/plugins/events-manager/