Call to undefined method K2HelperHTML::loadjQuery()

<?xml version="1.0" encoding="utf-8"?>
<extension version="2.5" type="plugin" group="k2" method="upgrade">
	<creationDate>March 17th, 2015</creationDate>
	<copyright>All rights reserved.</copyright>
	<description>A Plugin creating an extra field type to link to k2 items or categories</description>

	<files>
		<filename plugin="link">link.php</filename>
	</files>
	
	<fields 
		group="item-content"
		addfieldpath="/administrator/components/com_k2/elements/">
		<fieldset addfieldpath="/plugins/k2/link/elements">
			<field 
				name="lnk_item" 
				type="item" default="" 
				label="K2_ADD_ITEMS" description="K2_USE_THIS_TO_ADD_ITEMS_TO_THE_LIST_BELOW" />
      <!--
			<field 
				name="lnk_items" 
				type="litems" 
				default=""
				label="K2_DRAG_AND_DROP_TO_REORDER_ITEMS" 
				description="K2_DRAG_AND_DROP_TO_REORDER_ITEMS_CLICK_THE_REMOVE_ICON_TO_REMOVE_AN_ITEM_FROM_THE_LIST"/>
      -->
			<field
				name="lnk_only"
				type="radio"
				default="0"
				label="link only"
				description="If set to Yes, the item itself will not appear in any Category listing but its Link instead">
				<option value="0">JNO</option>
				<option value="1">JYES</option>
			</field>
    <!--
			<field 
				name="lnk_category" 
				type="lcategoriesmultiple" 
				default="" 
				label="K2_SELECT_ONE_OR_MORE_CATEGORIES" 
				description="K2_SELECT_ONE_ORE_MORE_CATEGORIES_FOR_WHICH_YOU_WANT_TO_FILTER_AN_ITEMS_LIST_SELECT_NONE_TO_FETCH_ITEMS_FROM_ALL_CATEGORIES"/>
			-->
		</fieldset>
	</fields>
		
	<fields 
		group="category" 
		addfieldpath="/administrator/components/com_k2/elements/">
		<fieldset addfieldpath="/plugins/k2/link/elements">
			<field 
				name="lnk_item" 
				type="item" default="" 
				label="K2_ADD_ITEMS" description="K2_USE_THIS_TO_ADD_ITEMS_TO_THE_LIST_BELOW" />
      <!--
			<field 
				name="lnk_items" 
				type="litems" 
				default=""
				label="K2_DRAG_AND_DROP_TO_REORDER_ITEMS" 
				description="K2_DRAG_AND_DROP_TO_REORDER_ITEMS_CLICK_THE_REMOVE_ICON_TO_REMOVE_AN_ITEM_FROM_THE_LIST"/>

			<field 
				name="lnk_category" 
				type="lcategoriesmultiple" 
				default="" 
				label="K2_SELECT_ONE_OR_MORE_CATEGORIES" 
				description="K2_SELECT_ONE_ORE_MORE_CATEGORIES_FOR_WHICH_YOU_WANT_TO_FILTER_AN_ITEMS_LIST_SELECT_NONE_TO_FETCH_ITEMS_FROM_ALL_CATEGORIES"/>
			-->
		</fieldset>
	</fields>
</extension>

require_once (JPATH_ADMINISTRATOR.'/components/com_k2/elements/items.php');

class JFormFieldLitems extends K2ElementItems
{
  public function fetchElement(…) {
    $out = parent::fetchElement(…); //that caused the error

    return preg_replace(…, …, $out);
  }
}