How to fix Error Undefined in Firefox?

Question

JavaScript

When deleting an item from the cart, an error Error Undefined appears - Firefox

'remove': function(key) {
    $.ajax({
      url: 'index.php?route=checkout/cart/remove',
      type: 'post',
      data: 'key=' + key,
      success: function(json) {
        
        // Need to set timeout otherwise it wont update the total
        setTimeout(function () {
          $('#cart > button').html('<div class="row d-flex align-items-end" style="position: relative;"><i class="fa fa-shopping-cart s1 col-sm-1 0"></i><span class="Notif">' + json['total'] + '</span></div>');
        }, 100);

        $('.modal-body').load('index.php?route=common/cart/info #contentCart');

        $('#checkout-cart').load('index.php?route=checkout/cart #checkout-cart > *')
        
      },
      error: function(xhr, ajaxOptions, thrownError) {
        alert(thrownError + "\r\n" + xhr.statusText + "\r\n" + xhr.responseText);
      }
    });
  }