Notice not displayed and shortcode braking the page

Hi,
Trying out your plugin. I need to display a message whenever the cart contain more then 12 plants of category “Tropical plants” or any of its subcategory. However the notice is not displayed on any page. I tried to add the shortcode to my custom Cart and Checkout page (https://tropicsathome.com/shopping-cart-checkout/) and the notice is still not showed and when I try to edit the page again I get the error message below:

Fatal error: Uncaught Error: Call to a member function get_cart() on null in /home3/astralex/public_html/tropicsathome.com/wordpress/wp-content/plugins/cart-notices-for-woocommerce/main.php:434 Stack trace: #0 /home3/astralex/public_html/tropicsathome.com/wordpress/wp-content/plugins/cart-notices-for-woocommerce/main.php(150): BeRocket_cart_notices->get_notice_list() #1 /home3/astralex/public_html/tropicsathome.com/wordpress/wp-includes/shortcodes.php(343): BeRocket_cart_notices->shortcode('', '', 'br_cart_notices') #2 [internal function]: do_shortcode_tag(Array) #3 /home3/astralex/public_html/tropicsathome.com/wordpress/wp-includes/shortcodes.php(218): preg_replace_callback('/\\[(\\[?)(br_car...', 'do_shortcode_ta...', '[kkstarratings ...') #4 /home3/astralex/public_html/tropicsathome.com/wordpress/wp-includes/class-wp-hook.php(292): do_shortcode('[kkstarratings ...') #5 /home3/astralex/public_html/tropicsathome.com/wordpress/wp-includes/plugin.php(212): WP_Hook->apply_filters('[kkstarratings ...', Array) #6 /home3/astra in /home3/astralex/public_html/tropicsathome.com/wordpress/wp-content/plugins/cart-notices-for-woocommerce/main.php on line 434
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