Can't edit orders. Blank page.

-- url of the page with the problem -- : www.frenchessentials.com/administrator/i...=order&task=edit&cid []=1475&cancel_redirect=L2FkbWluaXN0cmF0b3IvaW5kZXgucGhwP29wdGlvbj1jb21faGlrYXNob3AmYW1wO2N0cmw9b3JkZXI%3D
-- HikaShop version -- : 3.1.1
-- Joomla version -- : 3.7.5
-- PHP version -- : 7.2.16
-- Browser(s) name and version -- : safari 12.1
-- Error-message(debug-mod must be tuned on) -- : Fatal error: Declaration of JToolbarButtonHikapopup::fetchButton($type = 'Modal', $name = '', $text = '', $url = '', $width = 640, $height = 480, $top = 0, $left = 0, $onClose = '', $title = '') must be compatible with JToolbarButtonPopup::fetchButton($type = 'Modal', $name = '', $text = '', $url = '', $width = 640, $height = 480, $top = 0, $left = 0, $onClose = '', $title = '', $footer = NULL) in /home/o109775/public_html/administrator/components/com_hikashop/buttons/hikapopup.php on line 45

Hi,
When I'm in the dashboard and want to edit an order or create a new one it results in a blank white page and nothing.
Example of one order URL:
www.frenchessentials.com/administrator/i...=order&task=edit&cid []=1476&cancel_redirect=L2FkbWluaXN0cmF0b3IvaW5kZXgucGhwP29wdGlvbj1jb21faGlrYXNob3AmYW1wO2N0cmw9b3JkZXI%3D

the debug message:
Fatal error: Declaration of JToolbarButtonHikapopup::fetchButton($type = 'Modal', $name = '', $text = '', $url = '', $width = 640, $height = 480, $top = 0, $left = 0, $onClose = '', $title = '') must be compatible with JToolbarButtonPopup::fetchButton($type = 'Modal', $name = '', $text = '', $url = '', $width = 640, $height = 480, $top = 0, $left = 0, $onClose = '', $title = '', $footer = NULL) in /home/o109775/public_html/administrator/components/com_hikashop/buttons/hikapopup.php on line 45

How can I fix this problem? Anybody has an idea?
Thanks.