potentially fixes #5203

BTCPayServer/wwwroot/pos/cart.js

@@ -160,7 +160,10 @@ document.addEventListener("DOMContentLoaded",function () {
             }
         },
         mounted() {
-            this.$cart = new bootstrap.Offcanvas(this.$refs.cart, {backdrop: false})
+            const self =this;
+            nextTick(() => {
+                self.$cart = new bootstrap.Offcanvas("#cart", {backdrop: false})
+            })
             window.addEventListener('pagehide', () => {
                 if (this.payButtonLoading) {
                     this.unsetPayButtonLoading();