Warning on AJAX: htmlspecialchars() expects parameter 1 to be string in bootstrap.inc on line 1567

.

Here is $display value before generating the $settings via:

  $settings = array_merge(field_info_formatter_settings($display['type']), $display['settings']);

$display = array(5) {
  ["label"]=>
  string(5) "above"
  ["module"]=>
  string(13) "commerce_cart"
  ["settings"]=>
  array(5) {
    ["combine"]=>
    int(0)
    ["default_quantity"]=>
    int(1)
    ["line_item_type"]=>
    int(0)
    ["show_quantity"]=>
    int(0)
    ["show_single_product_attributes"]=>
    int(0)
  }
  ["type"]=>
  string(30) "commerce_cart_add_to_cart_form"
  ["weight"]=>
  int(4)
}

field_info_formatter_settings($display['type']) = array(5) {
  ["show_quantity"]=>
  bool(false)
  ["default_quantity"]=>
  int(1)
  ["combine"]=>
  bool(true)
  ["show_single_product_attributes"]=>
  bool(false)
  ["line_item_type"]=>
  string(7) "product"
}

$settings = array(5) {
  ["show_quantity"]=>
  int(0)
  ["default_quantity"]=>
  int(1)
  ["combine"]=>
  int(0)
  ["show_single_product_attributes"]=>
  int(0)
  ["line_item_type"]=>
  int(0)
}
$view_mode = 'default';
$instance['display'] = array(3) {
  ["checkout_pane"]=>
  array(4) {
    ["label"]=>
    string(5) "above"
    ["settings"]=>
    array(0) {
    }
    ["type"]=>
    string(6) "hidden"
    ["weight"]=>
    int(0)
  }
  ["default"]=>
  array(5) {
    ["label"]=>
    string(5) "above"
    ["module"]=>
    string(13) "commerce_cart"
    ["settings"]=>
    array(5) {
      ["combine"]=>
      int(0)
      ["default_quantity"]=>
      int(1)
      ["line_item_type"]=>
      int(0)
      ["show_quantity"]=>
      int(0)
      ["show_single_product_attributes"]=>
      int(0)
    }
    ["type"]=>
    string(30) "commerce_cart_add_to_cart_form"
    ["weight"]=>
    int(4)
  }
  ["teaser"]=>
  array(5) {
    ["label"]=>
    string(5) "above"
    ["module"]=>
    string(23) "commerce_product_bundle"
    ["settings"]=>
    array(3) {
      ["default_quantity"]=>
      int(1)
      ["show_fieldset"]=>
      bool(true)
      ["show_quantity"]=>
      bool(false)
    }
    ["type"]=>
    string(40) "commerce_bundle_product_add_to_cart_form"
    ["weight"]=>
    int(0)
  }
}

Whole $instance variable:

array(14) {
  ["default_value"]=>
  NULL
  ["description"]=>
  string(0) ""
  ["display"]=>
  array(3) {
    ["checkout_pane"]=>
    array(4) {
      ["label"]=>
      string(5) "above"
      ["settings"]=>
      array(0) {
      }
      ["type"]=>
      string(6) "hidden"
      ["weight"]=>
      int(0)
    }
    ["default"]=>
    array(5) {
      ["label"]=>
      string(5) "above"
      ["module"]=>
      string(13) "commerce_cart"
      ["settings"]=>
      array(5) {
        ["combine"]=>
        int(0)
        ["default_quantity"]=>
        int(1)
        ["line_item_type"]=>
        int(0)
        ["show_quantity"]=>
        int(0)
        ["show_single_product_attributes"]=>
        int(0)
      }
      ["type"]=>
      string(30) "commerce_cart_add_to_cart_form"
      ["weight"]=>
      int(4)
    }
    ["teaser"]=>
    array(5) {
      ["label"]=>
      string(5) "above"
      ["module"]=>
      string(23) "commerce_product_bundle"
      ["settings"]=>
      array(3) {
        ["default_quantity"]=>
        int(1)
        ["show_fieldset"]=>
        bool(true)
        ["show_quantity"]=>
        bool(false)
      }
      ["type"]=>
      string(40) "commerce_bundle_product_add_to_cart_form"
      ["weight"]=>
      int(0)
    }
  }
  ["display_label"]=>
  string(0) ""
  ["label"]=>
  string(14) "Extension pack"
  ["required"]=>
  int(0)
  ["settings"]=>
  array(3) {
    ["field_injection"]=>
    int(1)
    ["referenceable_types"]=>
    array(6) {
      ["consulting_services"]=>
      int(0)
      ["invoice"]=>
      int(0)
      ["foo_pro"]=>
      string(11) "foo_pro"
      ["product"]=>
      int(0)
      ["subscription"]=>
      string(12) "subscription"
      ["training"]=>
      int(0)
    }
    ["user_register_form"]=>
    bool(false)
  }
  ["widget"]=>
  array(5) {
    ["active"]=>
    int(1)
    ["module"]=>
    string(7) "options"
    ["settings"]=>
    array(0) {
    }
    ["type"]=>
    string(15) "options_buttons"
    ["weight"]=>
    int(6)
  }
  ["id"]=>
  string(3) "222"
  ["field_id"]=>
  string(3) "133"
  ["field_name"]=>
  string(29) "field_commerce_extension_pack"
  ["entity_type"]=>
  string(4) "node"
  ["bundle"]=>
  string(20) "product_subscription"
  ["deleted"]=>
  string(1) "0"
}

So the source of the problem is ["line_item_type"]=> int(0) somewhere in field settings. But I'm not sure which settings can be wrong.

My product display field (field_commerce_extension_pack ) is Product reference type displayed as Check boxes.
But the same happening for field_product_type which is Product reference displayed as Select list.
And even for General image (field_general_image) which is Image field.

Proposed patch:

--- a/modules/cart/commerce_cart.module
+++ b/modules/cart/commerce_cart.module
@@ -2537,7 +2537,7 @@ function commerce_cart_field_formatter_settings_summary($field, $instance, $view
       t('!visibility attributes on single product forms.', array('!visibility' => !empty($settings['show_single_product_attributes']) ? t('Showing') : t('Hiding'))),
     );
 
-    if (count(commerce_product_line_item_types()) > 1) {
+    if (count(commerce_product_line_item_types()) > 1 && $settings['line_item_type']) {
       $summary[] = t('Add to Cart line item type: @type', array('@type' => commerce_line_item_type_get_name($settings['line_item_type'])));
     }
   }