Warning after update to 2.0

-- HikaShop version -- : 3.2.1
-- HikaMarket version -- : 2.0
-- Joomla version -- : 3.8.1
-- PHP version -- : 7
-- Browser(s) name and version -- : Chrome
-- Error-message(debug-mod must be tuned on) -- : Warning: Parameter 1 to plgHikamarketVendorlocationfilter::onBeforeVendorListingDisplay() expected to be a reference, value given in P:\Programs\XAMPP\htdocs\globaltest\libraries\joomla\event\event.php on line 70

Hi all,
it's not a critical bug, it's just a new warning that came up after the upgrade to HikaMarket 2.0 and I wanted you to know.
It's within the vendorlocationfilter Plugin. HikaShop Plugin has no warnings at all.

Regards,
Fabio

Hello Jerome, thanks again for your quick reply.

You're totally right here, the call that seems to make problems is event.php in joomla libraries. I did do some modifications, that's why I also tried on a clean installation. Same issue. I'm not a php developer and I'm currently digging myself into this. I now think it's probably not a HikaShop issue but I thought maybe you'd see the issue right away.

The call which produces the warning is following:

return call_user_func_array(array($this, $event), $args);

/**
	 * Method to trigger events.
	 * The method first generates the even from the argument array. Then it unsets the argument
	 * since the argument has no bearing on the event handler.
	 * If the method exists it is called and returns its return value. If it does not exist it
	 * returns null.
	 *
	 * @param   array  &$args  Arguments
	 *
	 * @return  mixed  Routine return value
	 *
	 * @since   11.1
	 */
	public function update(&$args)
	{
		// First let's get the event from the argument array.  Next we will unset the
		// event argument as it has no bearing on the method to handle the event.
		$event = $args['event'];
		unset($args['event']);

		/*
		 * If the method to handle an event exists, call it and return its return
		 * value.  If it does not exist, return null.
		 */
		if (method_exists($this, $event))
		{
			return call_user_func_array(array($this, $event), $args);
		}
	}